No thanks. This would defeat the whole point of having predesignated roles and totally screw with Duty Finder queue times. And imagine the tears. "You suck at tank, I'm changing... Change to something else! What do you mean you don't have a Healer? /kickvote". Queue for what you want to do the dungeon on. It's not like you'll have to change inside because you'll sometimes be missing a role. Duty won't commence without them.

Why fix what isn't broke?