As someone has mentioned already, I actually really like how DF is right now with the restrictions because of the random element of running into people. Premade, however, you can still arrange the party however you want, but putting up restricted roles when randomly putting people together I actually very much like. In my opinion, it seems like a decent way of reducing queue hoppers who queue for the wrong role solely so they can jump ahead in line. I can't say that it works as a fact, though... just theory.