Multi-boxed DPS bring coordination to make killing a target very efficient, and quick. Speed is what they offer.

Multi-boxed Heals bring efficient healing, to make mana efficient, but they slow things down greatly. The longer the fight, the more time you give your opponent to figure out that you are boxing and how to beat you, especially since your damage output would be very very sad.

Multi-DPS works because it eliminates enemies fast. Multi-Healer is not this at all, and really, 5 individual healers would behave better when it came to CC.