I am one of "those" people who will leave before the game begins if an unfavoured dungeon pops up, so I will tell you what the deal is from personal experience. Kimura410 has got it mostly right. For players who already have everything there really isn't much penalty for skipping out, especially when the rage quit penalty got heavily reduced on patch 2.2 (i think). You have to leave a dungeon 3 times in a row under an hour in order to get the penalty. People are expliting it through roulette by rolling the dice twice, and then if they don't get the dungeon they want they just que for it directly without roulette. After it's complete they'll use the roulette and try again. This process is repeated until the favored dungeon pops up on roulette. The system is very flawed honestly. I remember back then when you got a 30 min penalty instantly for leaving. Kind of miss that system really.