I actually misunderstood what Duo was saying.
I no longer agree.
I like what Hyrist says. The roulette should unlock once all the storyline dungeons are completed. Then, perhaps give more of a roulette bonus (or a solid first-time completion bonus) for finishing each optional dungeon for the first time.
That way, the little quests to unlock those dungeons aren't trivialized, and people can still do those quests to unlock the dungeons, which is kind of a core component of a Final Fantasy game.