First, I dislike the OA martial arts styles. Nice idea, poorly implimented.
I seems to me that you just look at a prestige class as a way to get more abilities. Show me how any of these are not either totally munchkin or just lame. They make good NPC characters.
I can't prove an opinion. And that's all you have there is your opinion. For the record, however, there is not a one of those I would consider munchkin, and only one I would consider "lame" at all (I've never really liked the Dragon Mage)
And I'll thank you to not claim to know how I think, because you don't seem to.
In a sense you are right; a prestige class is a way to get new opinon. Honestly. I mean, who would take a prestige class that basicly just gave you ten levels of fighter? No one. There would be no reason to, even for roleplay reasons (You could then just say you were XXXXX).
But I don't see how getting new abilities is a bad thing. I really don't.
I really, really, really don't understand your claim that prestige classes only work for NPCs. Not at all. Not in the slightest. A handful are aimed at NPCs, yes, but not all. Not even most. Not even very many. In fact, I can only think of maybe five off hand (Of WotC published ones, that is). The rest work equaly well either way.
You also seem to have a somewhat unusual group of people around you... Prestige classes are fairly common talk among my groups. Not that everyone takes one or anything, but a lot of people talk about it, and some do.
And remember, focus (Narrow focus even) is what prestige classes were originaly intended to be for. That kinda got lost, but...
Furthermore, how would be be better off without prestige classes? That's the real question to consider. They are totaly optional. It's like saying we would have been better off without the Bugbears or Gnolls. It doesn't matter one way or another since it plays no core role.