I've seen this pitched before, but once you've zoned into an instance, the roulette will lock that player into that specific instance until it is completed if they leave and queue up for that roulette again. That way, people who leave simply because it's an instance they dislike have the options to skip that roulette for the day, or suck it up and complete that instance and the people who have a bad group/have an emergency/etc can simply leave and come back and finish it later for their bonus.