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......
I don't see the point in guarding healers, if the dps haven't picked up all the extra adds (i.e. in cases where you get two dps attacking same target instead of taking an add each) and the adds decide to go for the healer then a guard isn't going to matter much to that particular mobs threat as the healer is still going to be the only one that mob sees as a threat.