If roulettes (aside from 90 and expert) did not offer exp, I would basically never run them except on a 90 job where I could get tomes for them. If they didn't offer tomes, I would never run them except on a sub-90 job where I could get exp from them. If they offered neither, I would never run them. This would make leveling incredibly frustrating, because the only way to do it effectively would be to keep spamming the highest level dungeon and hope there were enough people queueing for it that you could spam it with (or alternately use duty support). They are the primary way to level for most people doing alt jobs and most people would not queue for them otherwise.