There's really no point to add another category for that at this point of the game since the closest we have to that is Ninja/Scouting since it scales off of dex while still being magical. And even if they made DRK into that role they more than likely would not make it sacrifice hp to do anything, They removed all the "increase damage to self" from normal jobs and that would fall under it. MP is just the HP and there's not really a reason to change that.