It is an interesting point being made about weight classes. In boxing training generally you don't really put too much thought into the weight class of your sparring partners and the same is true for judo. In the latter, where I was weight class only started playing a role in grading competitions after nidan. I would think that there is good reason to train with bigger and stronger partners no matter if you goal is competition or self defense.
Oh, I agree. But one of the things you learn by doing that is that strength and (physical) power don't help much if the other person has more of them, and therefore relying on them in a self defense situation is extremely unwise.