Intriguing thought. It would definitely be interesting to see SE implement a system that forced players to have a more mixed PT setup.

I agree with your statement about how things are doable with a setup of just about every class available currently, the problem is that it isn't efficient enough to risk losing. People have a hard enough time getting the drops that they want in the current end-game content that they are less inclined to mix it up. It's all about using the easiest classes to play in Ifrit and Moogle to insure the win. Unless you roll with a regular static group that are good enough to try different set ups and there are a lot of people that do.

I'm not going to complain about how it is now though because to be honest, I don't really care how many ARCs or THMs I have in a PT. Since 1.20 I've played every one of my classes(except LNC) nearly equally in all the end-game content. As long as they keep balance among classes I will play all of them where I can, whenever I can. It would be interesting to see if SE does something about class stacking, but either way I'm good.