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......
There is no point to ever guard a healer in PvE if the tank and the dps any clue on what they should be doing. Also in almost all cases, when healer does have aggro, having a guard would not really help.
Guard reduces threat generated but it really does not make any difference if someone has 10000 threat or just 8000 threat, if he is the only one to have threat on a specific mob, he will still be on the top of the threat list.
Tanks and dps will generate threat based on the damage they do and will have threat only on those targets they have damaged (not counting taunts etc). Healing threat is spread around to all the mobs you are in combat with.
Healer will ALWAYS have aggro on any and all mobs that neither the tanks or the dps are not doing any damage on, regardless of whether or not the healer has a guard on him.
Healing threat is much lower than threat caused by doing damage (even without any threat multipliers).
Healer will NEVER have aggro on any mobs that the tank OR dps are doing constantly damage on, regardless of whether or not the healer has a guard on him.
In either case, a guard on a healer will do absolutely no difference what so ever. There are very few situations where I can think of guarding a healer would help at all.
If a healer ever has aggro on him, the solution is not
to put a guard on the healer. The solution is for the tanks and the dps (if it is a trash group with standard/weak mobs in it too, the dps are as much responsible as the tanks) to learn to hit all the mobs they are in combat with.