Most quests earn you about 30k experience. At this stage in my white mage (level 56) I need approximately 2.6 million experience points to reach my goal. It would take 87 such quests to hit my next level.
Story quests give a nice amount of XP, but those are a one-off so they're not going to be available for other jobs I might level. That basically leaves the level 51+ dungeons to get experience in which is just ridiculous. As for the roulette bonus, while nice it's only once per day, so it's never going to be your main source of experience in the game.
Let's be honest, there's no good reason why the level 50 dungeons shouldn't give experience points. Their only reasoning behind it is to get us to do the newer content. An argument which falls flat on its face when you realize that only level 51+ can actually participate in any of it, so any current level 50 jobs are going to be grinding lowbie dungeons regardless.