Just because the Dark Knight's resource base is going to be MP doesn't mean it won't have a way to regain mp in a very quick fashion. I'd imagine they probably have a more potent version of the PLD's Riot Blade for MP gain. This would be game breaking if they had access to Flash, but I also suspect that one of the major drawbacks of these "Jobs" is that they won't get any cross-class skills, which will prevent them from getting Flash. I'm pretty sure MP management for Dark Knights will be very similar to the MP management for Black Mages.