In my opinion the rewards are made for fomenting the tanking and healing... And it works. It is my case that everyday I do the roulette I join with my marauder or gladiator, until the point my marauder became my main class and eventually I even enjoy it...

But I guess it depends of opinions and tastes, for example when the reward is for a healer in guildhests, I may take healer because is short thing. But a low level roulette... No, I can't imagine myself using healer more than ten minutes.

About the 30 minutes or more when being a DPS... Well. I hunt FATES or craft when waiting. Also you can do with a friend or FC member the smame this noob does... One dungeon your friend uses tank or healer and you DPS, the other dungeon vice versa. That way in party the duty comes fast for a DPS.