Queueing for roulette is like queuing for multiple dungeons at once. Not sure I get what the difference is. Imagine when you queue for leveling roulette, you have essentially queued for every dungeon at your level and below. You are placed in a queue based on your role. As you move up the queue, sometimes you are jumped over because your level is too low for the instance that someone else has queued up for. But you maintain your position, just everyone behind you moves up by 1.

It would be a lot of work to rewrite DF to be able to queue for 4, 8 and 24 man instances all at once.