"When they're not", that is basically a situation where nobody is doing any damage on those mobs that are outside the range of the aoe. Then healing is the only thing generating threat.
That is not healer pulling off. That is healer being the only one generating threat on those mobs.
I'll concede the point about the Guard Drouk, I hadn't considered the aggro reset.
But this is my point - with mob groups like that (of which there are many), the tank will have to split his attention between the ones in AOE range and the ones who are not. This is easy for a Powertech or Vanguard. I don't know about Assassins/Shadows. It's not so easy for a Juggernaut/Guardian. A good tank *will* be throwing attacks and taunts at those outside his normal range, but there's a limit to what can be done and splitting the focus like that decreases the amount of threat the tank has on those mobs.
With decreased threat, it's more likely that the healer will pull off. There's a difference between a Jug tank hitting a whole group with Smash + Crushing Blow + Sweeping Slash and a Jug tank having to bounce back and forth between doing that and running over to the other mob to choke him, running back, throwing the saber... etc. The healer, meanwhile, is generating the same amount of threat on all of them, and can easily pull off whichever one the tank has the least threat on.
In an optimal group where everybody knows what they're doing, nobody will pull off for long enough for it to matter (DPS might pull off, but probably only a second or two before they kill their target and will take minimal damage). Rarely is the situation optimal, and people make mistakes. The Guard is insurance for this, and that insurance is most important on the healer - if things go out of hand and people start dying, would you rather lose the DPS or the healer?
You're all talking like it can NEVER happen. People aren't perfect, stuff happens, and it's best to do what you can to minimize it.
Besides, on most trash pulls the DPS aren't starting on the tank's main focus anyways. They're attacking healers, weaks, normals, strongs, then elites and champs - and the guard's not going to make a difference THERE, either, as the DPS will still pull the weaker mob. By the time they get to the guy the tank has been building aggro on, he's so far ahead they won't pull off.