As someone who played a caballed healer semi-recently (Yikaro), if I didn't have the ability to heal outers with mass heal I would have gouged my eyes out with boredom.

Mass heal is a 2 round lag heal, so it's much more risky for the healer, and it's not as powerful as a rejuvenate. It takes a long time to heal a cabal guardian up like that, but at least it lets the healer participate in a defense.

Healers need to be good at something. They're already pretty unattractive to play as it is due to their lack of independence.

From my understanding, the modifications to heal were made because before that, a well played and well decked healer for Fort or Empire could basically shut down a retrieval from anything short of 3 lvl 40 chars. So, you don't want it to be totally hopeless for the non-heroes. But you also want to give the healer some purpose. Mass heal seems like a good compromise there.