So instead of an arbitrary number, you just want raid leaders to use some other arbitrary criteria? While it is a little more personal, it still suffers the same problem in that it will exclude people that could be an benefit to the group and it will include some that will have to be carried. It is just a nicer way, but slower way get to some meaningless minimum standard.
No. When I form groups that's what I do, I ask them how they're geared and how experienced they are and then make a judgement call based on that in combination with their class. I don't set any arbitrary "you have to be in this level of gear", I might take in a experienced marauder in columi but turn down the rakata geared assassin, or even the campaign geared merc who has never done the op before if I don't feel like teaching.
I know the classes pretty well, like any good raid leader should, so combining that with their experience does allow me to make a pretty accurate call on if they're a good fit for the group or not. It's not perfect, but it's a lot better.