I would simply argue that not every strike needs to be debilitating in order to be effective. Most jabs aren't knockout-worthy, but the jab remains a critical piece of any effective boxer's arsenal. The inside leg kick does damage.
Further, the inside leg kick is an important weapon to attack the opponent's footwork and disrupt their planned steps or kicks.
Evidence from Kyokushin Karate
This sherdog post is an excellent breakdown of how the technique can be devastatingly effective. It includes gif highlights from Kyokushin karate. His argument is that the inside leg kick can:
- cause damage when applied to a supporting (weighted) leg
- be disguised as an outside low kick to draw a check with the opponent's opposite leg
- off-balance or knock down an opponent by exploiting or forcing poor footwork on their part
- counter an opponent's charge
- counter an opponent's mid- or high-level kick
Evidence from MMA
This AintNoSunshine post argues that the inside leg kick is a powerful way to set up head kicks or mess with an opponent's footwork:
In his fight with Kenny Florian, Diego Nunes, despite taking the loss, showed how effective and dangerous an inside leg kick could be...
As you see, Florian’s right leg is leading as he stalks Nunes. That is the leg targeted most often, as kicking the rear leg leaves you more vulnerable defensively and takes a little more effort with less guarantee of effectiveness.
While Nunes is backpedaling in a subtle manner, he stops and sets his feet. While planted, Nunes swivels his hips thrusting his rear right leg forward in a slicing motion, landing right beneath Florian's knee. Part of the reason it lands so low is due to Florian checking/defending the leg kick. By picking up your leg, it helps absorb the strike in comparison to leaving it planted as it is kicked.
Nunes kicks Florian to his knee in a moment of unbalance and forces Florian to scurry up and retreat, resetting himself and stopping his stalking of Nunes.
Florian is once again stalking Nunes, walking him down while diligently looking for his opening. Not even a minute earlier Nunes caught the inside lead leg of Florian causing him to back pedal.
This is where that earlier strike gets paid off.
In the same manner as before, Nunes stops his retreating footwork and plants down to unload a kick. This time Florian is aware a kick may be headed his way, however, he assumes the kick will be attacking his leg.
He drops his posture as if to catch the kick and in turn, fails to protect his head. Nunes, instead of attacking the leg, goes upstairs for a head kick, landing flush on Florian's face and stumbling him backwards to once again retreat and reset.
By threatening Florian with the leg kick that threw him off balance, Nunes set up the head kick perfectly. Having Florian defend low enabled a high strike to land cleanly.
Furthermore, not to pick on Kenny Florian, but Jose Aldo also used an inside low kick to disrupt his opponent's balance, do damage, and set up subsequent strikes.
Evidence from Muay Thai
This is the damage taken by one student from just a few rounds of sparring someone who was keen on inside leg kicks and wasn't wearing shinpads:
Doesn't look like fun. The opponent now hurts, is distracted and worn down from that hurt, and is weaker and slower using that leg. Those all sound like effective uses of a strike.
The inside leg kick is used to damage the opponent, to counter their footwork and kicks, to disrupt and slow their footwork, and to set up head kicks. It is hella effective.