Well it, as you said depends on the game. Is every henchman evil, is every monster evil or every fantasic creature of the wilderness? If the only thing you ever fight in the game is evil than yes that is not an appropriate defence. I think though that most people will realize that the fight a lot more than just what is considered evil in their world.
I've a hard time believing that anyone plays in such a world, where you only fight "EVIL" enemies. Even in D&D most people are supposed to fall into the nuetral alignment (Though this has failed some what in practice in their materials), with only the truly virtuos or vile falling into either extreme.
That is why whenever you set these types of lims or advantages you need to look at your world.
I've always seent he term common defence or set of uncommon circumstances to mean "Reasonable". Obviously this is just my interpitation. So for NND Gas attacks, if someone has a rebreather or a gas mask or doesn't breath than they aren't effected. Where as if someone writes up a power that is say an NND Kryponite blast defense of not being kryptonian, you should realize they are telling superman that the defense is actually, "not being you" (he is the only kryptonian) and that this power write up may be inappropriate.