"You can't run out of MP" is overstating it somewhat. In normal play, it's mostly true. But if you die and then have to raise 5 people, it gets ugly.
Put my WHM on a fight like O8N and it can go on for 20 minutes with me flinging rocks every free GCD, and I won't run out of MP. Lucid and Thin Air are just that good. Bonus points if there's Mana Shift coming. :D
