Here is the imbalance

Pick up mobs -> "Hallowed Ground", "Sentinel" -> EZ kill the mobs -> switch and comeback to PLD, go to next mob (Like 2 sec. to do) -> "Hallowed Ground", "Sentinel", other CD available again -> EZ kill the mobs -> ..... indefinitly ?? don't think that's a good idea

Edit : I take the PLD example cause i am one but i'm sure there would be other exploit like that with other classes CD like damage up or other things