I guess it largely depends on what the overall attitude of groups is. It makes sense to ask for understanding the mechanics and enough dps to beat the enrage. But everyone has bad days, and lag an other issues happen. So I fear groups would start asking for significantly more dps than is necessary to provide themselves with a cushion against those random factors. If you only want to play with other elite players, you should make a static, not put up a PF.

Of course, if someone is having a bad day, or is too drunk to play, or whatever, the polite thing to do is to leave the group and not drag the others down. Or not join in the first place, but you don't always know these things beforehand.