In the wake of the most recent PTR change to Flametongue Weapon applying a -50% healing debuff over 5 seconds and -- it now appears -- the Flametongue totem itself doing the same for others' melee attacks, a number of forum threads have popped up questioning the increasing number of these debuffs in the game. The funniest asks, "Is there some sort of Mortal Strike non-proliferation treaty that stops me from having Mortal Strike on my priest?" (short of Hex of Weakness, I guess). Suggestions include an MS effect on Crusader Strike, "MORTAL SHEEEEEEEEP!", and "Mortal Portal" for mages.
The best argument I've seen is not that Mortal Strike or MS-like effects like Aimed Shot are themselves imbalanced, but they're bound to seem that way if healing is overpowered in PvP. Healing per second is nearly always more efficient than damage per second if you're specced for it, although that's cold comfort to yours truly while resto-specced and under heavy fire in battlegrounds or arena. Nobody knows if the newest version of MS is really going to help Shamans in arena, but between this and the nerf to drinking, it does look more and more as if PvP is increasingly being balanced around the notion of healers staying exposed (and vulnerable) for longer.