I really can't speak for black belts. I've never used one. I'll reply in an orderly way:
Fighter: Rocks. I agree.
Thief: He's actually an alright attacker, too. Just make sure you bring along a fighter.
Black Belt: I'm not qualified to comment. From what I've heard, he sucks.
Red Mage: I love these guys for support. Agreed.
White mage: agreed.
Black Mage: Agreed. I can't see why people hate them. They have their uses, and with a cat claw, their damage is actually decent.

So, yeah. I agree with this thing.