First off, a healer healing/sancing mobs is much less dangerous than a shaman summoning and plaguing. Generally if the healer cares that much, you're likely fighting good align mobs and the area isn't as dangerous. The shaman can summon/plague a bunch of assisting mobs and get you killed.

Second, a healer has to actually sit around for a while to grief you. A shaman bunching up mobs and getting you plagued takes a very small amount of time comparatively. The healer is a sitting duck for someone you call in, especially if they're following you all over. Meanwhile, a shaman can theoretically ruin an area for ranking, get you plagued or even killed, and gtfo in no time. Add outlander powers to all this and it's even more of a difference.

Even in your assumed scenario (which isn't what this conversation was about), it's nowhere near the same kind of grief.