As a pvp Vanguard, I am VERY reluctant to put guard on healers now. The healer will still die under focused fire and I wind up half-dead because he's too busy trying to keep himself alive to keep me topped off. I'm almost better off NOT guarding the healer because at least I'll be at full health when he goes down.
Remember, only one tank can guard a healer but there's no limit to the zerg. Once a healer is marked, there's nothing I can do to keep a him alive against 4-5 Marauders that instantly jump him.