Only generated party, but it has to be party of the sufficient size - 6 or more characters.
With party of 4 there is really no choice. You always need one frontliner, one healer, one magician and one lock-opening and trap-disarming bot, so all positions are occupied from the start.
I agree with your statement but disagree with your reasoning. I like when there are more important functions and desirable roles than slots. I think rpgs work best when every decision comes with a stiff opportunity cost. And my favorite part of my favorite rpgs is figuring out how I can get everything I want whittled down to everything I really, really want while hoping I didn't make too many bad decisions.