If you do roulette, and it's not the one you want, if you leave then you should be penalized. That's not how the game of roulette works.

If you leave: you forfeit the daily bonus, and are penalized soldiery. So, if you go a dungeon on roulette but leave because you want Brayflox, then your soldiery cap for that week is now 450-45 or 405. All myth tomes should drop at the final boss too, but that's another thread.