When we teach basic bunkai we often show impractical techniques and unrealistic situations for example blocking a mae geri by turning into the kick and using gedan barai. This is obviously not realistic for sparring or self-defense so why do we continue to teach these applications to beginners?
The answer is simple: Because the vast majority of karate is taught by instructors who don't know what realistic bunkai is. And that's because their instructors were never taught it. And their instructors' instructors were never taught it, etc. This goes back many generations.
I gave a good overview of the subject at this link.
And you should also read this link.
Read those two links first, and then continue...
Bunkai is useless without first understanding classical jujitsu or something similar. I'm talking about standing grappling as it is applied to self-defense. Because, as I explained in the links above, that's what kata have in them. It's not a bunch of blocks.
The bunkai that anyone comes up with on his/her own is just a reflection of their own knowledge. If all they have is an understanding of punching, kicking, and blocking, then that's what they're going to see in the movements of the kata. It's just that simple.
When people are taught classical jujitsu for a year or two, then things in the kata start to make more sense. Sometimes, like in the Heian kata "hands on hips" example I linked to above, it is obvious that it can not be anything other than grappling. And once you understand that, it's like a light will go on in your head, and you'll realize just how unrealistic and worthless the punch/kick/block interpretations were.
You don't learn how to block from doing solo kata. You learn that quite well in free sparring.
And vice-versa, you don't learn grappling based self-defense in free sparring (because "free" sparring means you are free and not holding your partner at all). You learn that in kata and partnered kata bunkai. At least, that's what you're supposed to learn from kata.
So why is the state of karate and karate bunkai this bad today? Why do most karate schools not teach proper bunkai?
Well I answered that already in the first paragraph and in the first link, above. It's because there are multiple generations of instructors who have not been taught it.
This situation would not be an issue if there was a set of standardized applications given for each technique in a kata taught along with the kata. Bunkai wouldn't even be necessary, because there'd be no interpretation and no guessing required.
The original authors of each kata had their own applications which, according to oral histories, they did teach to their top students (not all of their students, by the way). However, the transmission of that was incomplete and eventually faded away in pretty much all lineages of karate. No karate instructor these days can definitively say what each technique in their kata was originally designed to do.
The way to get it back and to reclaim the kata is to learn classical jujitsu and reverse engineer the movements in the kata. Or create new kata based on classical jujitsu and an understanding of the original karate kata. And then from there, it needs to be passed on as an official part of the system to all students, not just a trusted few. This is something that's obvious to me, and it's strange to me that karate systems haven't gotten their act together on this yet.
Hope that helps.
This is a complex question deserving a complex answer. In your question, you explicitly refer to gedan barai as an example of a generally "bad" bunkai. In this answer, I will start from there to explain why it is taught that way, the purpose behind it, and why it appears to be missing something.
1. Gedan Barai is not a block.
In karate, blocks are generally called "uke", as in "jodan uke", or upper block. Uke means "to receive", and generally represents the act of absorbing most of the impact during the block, or at least striking the attacking limb.
Barai means "sweep", not block. In gedan barai, you are not supposed to block the attacking leg. In it's simplest application, gedan barai is intended to move the leg one way while you go the other way. The intended application is for you to step back (and not "turning into the kick", as you mention in the question) into a zenkutsu-dachi and deflect the kick so it lands beside you. If performed correctly, you should essentially be standing behind your attacker.
This leaves your opponent open for a yoko geri to the knee, or a plethora of other counter attacks. My favorite, personally, is to use the blocking hand to transition into hadaka-jime, a form of choke. But, had I not cross-trained in judo, I would likely have never learned how to execute it properly.
2. What you call "basic" bunkai is just the simplest way to visualize the principle of the technique.
Gedan barai is first learned to defend against a basic mae geri. This serves two purposes:
- It helps the uke practice his kicks, learning proper maai so that, unless the tori moves out of the way or successfully blocks/deflects, his kick would land solidly. This is probably the most important part of a partnered drill, otherwise there is no reason to block in the first place.
- Tori learns that the most important step is moving out of the way. The actual gedan barai is an offensive move meant to manoeuvre your opponent into a less desirable position (i.e. showing you his back).
While the technique itself is fully applicable, it's just the tip of the iceberg. Once the student understands the principle behind gedan barai (see #2 above), he can apply it to other scenarios, and advanced techniques can be grafted to it.
Here is another short video showcasing a few different applications. Note that these are purely demonstrative versions and, as such, it should be understood that while you do start drilling these applications in the same way, you eventually transition to more robust drills with an opponent that keeps attacking. If not, the problem lies with the sensei, not with the technique itself.
3. Most dojos stop there because they never learned what comes next.
Speaking of problems lying with the sensei... Once you reach that point and understand how to use gedan barai to properly defend against punches, kicks, grabs, attempted grabs, etc., now you are ready to learn how to use this technique to transition into tegumi or mutō, known in occident as wrestling.
There are multiple levels of bunkai and, because no one really knows tegumi or mutō outside of a select few people, the last levels are not taught anymore. As I mentioned in an answer to a recent question, ancient karateka were expected to be proficient grapplers and would routinely cross-train in both karate and Okinawan wrestling. Karate was exported to Japan and the rest of the world, but Okinawan wrestling was left behind to reduce the risk of injuries. As such, modern karateka generally only know the basics of their techniques, but not the most advanced of their applications.
This is partly why I now practice judo. While karate can take me to grappling (my karate dojo still practiced tuidi applications), I had no idea what to do from then on. A lot of people have gone the BJJ route, but I feel like the judo throws mesh better with karate, as their main objective is to keep you standing while leaving your opponent down. In a self-defense situation, that's my cue to leg it. It's a matter of preferences, really.
But does it mean that karate alone is completely useless? Not at all! The basics can still take you pretty far. In fact, a lot of karateka compete in kickboxing events with great success and, while it's not 100% representative of a real life or death street fight, it's at least a proof that it works against trained and resisting opponents.
The answer is simple, and I'll quote a familiar phrase to illustrate:
"And some things that should not have been forgotten were lost. History became legend. Legend became myth. And for two and a half thousand years, the ring (I mean, technique) passed out of all knowledge."
And therein lies your answer: Children.
Children are not taught such bunkai because it was not part of their curriculum, or, the concept is too difficult for them to grasp. So, a question arose: "If we're not going to teach them true bunkai, what will we teach them?"
What is taught is a "visual". With a visual, we can get a student to turn into an incoming roundhouse or front kick coming at us from the side, and "block" it. Without this visualization, it's nearly impossible to get a child to remember the proper execution of the actual bunkai or of the technique in the kata. In this phase of his training, you get the kid to DO the technique, and next phase of his training is to get him to UNDERSTAND the technique. And therein lies the catch-22.
The Japanese have terms for this concept: "Bunkai Omote" (obvious), and "Bunkai Ura" (hidden, secret, or alternative). What we start out teaching children (and perhaps beginner adults) is the bunkai omote, because it's obvious and simple.
Back to the Lord of the Rings...
That child you just taught was a turn into a kick and block. At some point in his life, someone is going to have to explain to him that all that he thinks he knows just isn't true.
In the case of fanciful bunkai, that child grows up never being taught the true bunkai, either his school does not have the bulk of instructors who can pass on that knowledge, or, it has already become a bulk of instructors who were that kid who was taught the visualization. What the kid knows becomes myth, and that myth becomes legend, and like any good legend, it gets passed on. It makes sense, after all, yes? "My instructor taught me that, so that must be the case, yes?" He will eventually will become an instructor who believes that is what the bunkai is all about, and thus perpetuates this myth and legend.
In some schools, kids may be taught that visualization and later, if he sticks around, will be taught the ura bunkai. "IF". And if he doesn't, he loses out on that knowledge, or, he perpetuates it by starting his own school, or joins with another with the same background.
This explains why there are some schools that teach it properly, and why there are some schools that just don't get it. And such fanciful legends, like the colors of the belt, the spearhand, the crane stance, the do-not-let-the-uniform-touch-the-floor, these become rules of the style, and everyone subscribes to them - not realizing they're all just stories we teach kids because they're good visual aids. Haven't we all "lied" to a kid in order to effect a benevolent moral concept, or to explain a difficult concept?
And, the explanation doesn't end there, either. This will continue until the student questions what he is taught. This will happen as soon as he realizes "something ain't right". But in many dojos (I can tell you this is rampant in Taekwondo), it is taboo to question the instructor, and so, that student doesn't always get that chance to question what he is taught, thus cementing this legend's fate. Luckily, there is the Internet, and everything becomes exposed.
There is a surprising example of how this legend propagates from child to instructor to grandmaster. One need only read a copy of Gen Choi's Taekwondo Encyclopedia. In it contains many fanciful explanations of kata (therein called hyung, or tul), some so implausible as to be a comical farce. And that is ITF, a very different branch of Taekwondo than Kukkiwon style Taekwondo, and, Kukkiwon is not immune from this, either. Such has an entire university, a vast library, and pushers and movers of the style, some with doctorates to their credit. And guess what? "That there is a turn to the left, block a kick, step and punch".
And then the next reason why we teach this:
Yes, it's useless and irrelevant.
Looking to Taekwondo, it is a simple idea that we will need to block a kick more than we will need to perform a grappling technique, because, in the style, there is no self-defense application. (Go ahead, read the Kukkiwon Textbook or Gen Choi's Encyclopedia... there is scant details about specific self-defense functions in most of the techniques, and much of it is fanciful).
Looking to Karate, I wouldn't say it's useless unless the school focuses mainly on competition and sparring, because without a self-defense curriculum or a curriculum which places emphasis on grappling, the bunkai omote is all that you get. And for those in that style or brand of school, that is all that they need to know. They'll never apply any kind of bunkai - omote or ura - because it's just not part of their curriculum.