tl;dr you want to make queue times shorter by making queue times longer for other people. The only dps who'll actually hit roulettes will be those who already run tanks and therefore enjoy faster queues to begin with.
Also, with how painful dps queues will still be, no one's going to spend time running a roulette on a class they don't want to level, and then hitting the dps queue which likely will still be painful. Not everyone has that sort of time.