Personally, I don't see how it is encouraging any more participanting if pre-made team can do roulette. I start doing my tank class and queue with my friend as DPS so that she won't be queue forever. We queue for dungeon via DF (non-roulette). Because we don't get the roulette bonus, so it will take more dungeon run for us to level up. As a result it makes me go into dungeon more often as a tank and fill this "much needed" role for other people.

I don't see there is an issue with the current roulette implementation either. I think the exp bonus is there for those who has courage to let themselves to go in a random party without knowing anyone prior. It is BONUS, but not a given. People who doesn't have this patience with other people and lacking social skill can work harder to get the same experience. You can always have pre-made team to dungeon There is nothing in the game that stop you from playing with people you know, but you just can't level as fast.