As far as incentives and the roulette goes. Incentives are necessary for the roulette to exist, and for queue times to remain remotely bearable (as bearable as they can be). When you queue for a specific dungeon, you are "requesting" that dungeon. When someone queues for the duty roulette, in theory they will "fill" that request assuming it falls under that category. Without incentives nobody will run roulettes, because the optimal dungeons are specific dungeons based on rewards. Thus they will only ever pick the dungeons with the most "value". Imagine if all level 60 players stopped running leveling roulette. Suddenly your queue times would shoot through the roof, so to speak.