Sequential Partnered Drills
As you mention, drills where you're not actually facing multiple opponents simultaneously, but rather sequentially. The value in this is both endurance training and the issues of having to adjust to different, fresh opponents.
Of course, this is not the same as dealing with multiple opponents simultaneously, and so, the value you get is not the same as multiple opponents. Most of the time, I've seen schools do this either out of an issue of size and space, or because there's too much variety in skill levels for safe control.
True Multiple Opponents
Many schools focused on combative aspects do this training and it's a world of difference between single partner and multiple opponents.
- Time - you don't have time to "pick apart" opponents when it's a group
- Movement - you have to constantly find a way to not be rushed at the same time
- Grappling - you want things you can do that keep you mobile, not tied up on the ground
- Awareness - you have to pay a lot of attention to your terrain and where everyone is
Even if you find yourself in a situation where it's 4 vs. 4 or something, nothing guarantees everyone is going to pair up 1:1 in a fight. So these skills become critical. Here's a video where they took some MMA fighters through Marine hand to hand training - starting at 3:50 is where you can see an exercise where they end up doing not so great because the Marines double team, take down one, then go to the next.
For the people who are being the "multiple opponents" they end up learning some useful skills as well:
- How to find an opening when an opponent is distracted
- How to maneuver around bodies to get to a target
- Attack options when you literally have extra sets of hands
The difficulty in training these things comes from a few factors:
Depending on the skill level and what you're aiming to do, you want to set an agreement of intensity for this particular exercise. For example, it may make sense to stick to dealing with multiple opponents only doing 1 or 2 types of striking attacks. It might make sense to train for takedowns, etc.
The reason this is important is that if there's no boundaries set initially, maybe one person goes for a hard takedown/throw, the other person wasn't expecting it to get that serious, and they start going aggressive out of ego/stress response and it gets everyone going harder than they should until someone gets hurt.
As the skill levels go up, you can increase intensity, with communication being #1.
The other part is that because multiple opponent training is more unpredictable - for everyone involved, the single person or the group, it becomes very weird in terms of finding yourself in angles of attack or defense you didn't expect - which means your techniques are going to be stressed in weird ways.
Which also means if you're doing something that is more likely to cause accidental injury (takedowns, grapples, locks, some weapons, etc.) you want more control in this exercise on a technical stand point.
Training for a world where hurts count
This is true of a lot of training, but especially multiple opponents. Part of setting intensity is figuring out "when do we accept an attack/technique counts, even if it's being held back on for the sake of safety?"
If I'm hitting an opponent multiple times in the neck with a rubber knife, and he's pretending it has no effect, and continues to grapple me, while other opponents run up on me at the same time, what have we trained for? He's trained for a world where he's magically invincible, and I've trained for a world where knives to the neck don't work. And the problem is, these may be options that are exactly what you'd be doing in a multiple opponent situation.
So it's useful to say "These techniques can be used this far, and if this happens to you, you should give and consider it successful."
This is not to be mistaken with the schools that drill and assume all techniques instantly work ("Then you punch him, and then he falls down. GUARANTEED."), but it's critical because what happens when you DON'T have this is that your training falls into near full contact or grappling, because everything else "doesn't count" and you find yourself reducing safety AND not training some of the more useful options at all.