Right now i select a dungeon and then i choose an appropiate character. Given the choice it just feels awkward to take something like a halfling rouge. It's not that i don't want to play a halfling rouge at all, but i rarely feel like making an inferior choice right at the moment i have to choose.
This would change alot if those strange combinations would be given by the game. I would view this as a challenge instead of a dumb move. I know it's just psychological, but it would have a refreshening effect for the casual dungeon runs for me.
One could also incentivise going random with a slight starting gold benefit, starting piety or instant Jehora Jeheyu worship. In beta this could increase the play of less effective combinations in order to find more bugs.
(actually halfling rouges make half decent use of reflex potions, if you survive one hit - so replace halfling rouge with any bad combination of your choice