Hehe, yeah, that's almost as annoying as when a dps yells at a healer to heal the tank when their health gets a little low. Glad you took the time to notice someone else's health bars while you're dps'ing, that's pretty much what I've been doing all night.
In lower levels I think guarding a healer is a good idea, seems like everyone just has less tools to manage threat and tanks have less taunts. At level 50 I don't really understand why most tanks seem to want to just guard the healer, though.
Tank guards DPS during Boss Fights
Tank guards Healer during Trash pulls
If a DPS needs to be guarded during Trash pulls they just suck......