You wouldn't do it if you only had one Disciple out, but if you had 3-6 out, a healing plus 3-6 damage works for 5 mana. And it's only 3 mana really since you have to assume the charm's already been cast so the original 2 mana would have been a round or 2 ago. Plus the added benefits of 1 extra life for revealing charm, the possibility of it costing less with Enchanter's Ring, Ring of Asyra, etc.