No, you cant see which duty you'll get when going in with the roulette.

To answer your second question: I've got two theories - due to long queue times, a dps is more likely to let the timer run out because they went to do something else. And second: Say you have an hour on your hand, which is enough to run a dungeon, so you queue up and then... the queue takes ages. You're nervously keeping an eye on the clock, maybe begin to get ready for work and when the queue finally pops you have to choose if you can make a dungeon before work or not.