You show me a player that insists that guarding the healer is the best thing to do, and I'll show you many more that will argue with math why it isn't (for most fights).
Skornlord is 100% correct. Healing threat is 1/2 of effective healing, divided among the mobs present in the encounter. Also, note that effective healing does NOT include overheal.
Here are some of the main the reasons why many players continue to insist that guarding the healer is the best way to go:
1. An unguarded healer is getting attacked by mobs and thinks it will help.
2. The healer getting hammered by mobs wants the 5% general damage reduction.
3. The player misreads the tooltip and thinks the 25% damage transfer works outside of PvP.
Now, the healer getting attacked by mobs is not due to a lack of guard. It's because NOBODY ELSE IS ATTACKING THE MOB
. Therefore the healing threat is right at the top of the aggro table for the mob in question.
Let's look at the math of how easy it is to keep aggro away from a healer.
Take the first big pull in HM FE. 2 sith mauraders, 2 big droids, 3 ranged in the back. Let's just pretend for the sake of the argument that you're healing 1500 hps. 1500 * 50% / 7 = 107 threat per second. A dps can beat that with occasional free attacks while having tea and scones with his teddy bear.
It's simple. If the healer has aggro, the other players in the group are not doing their job.
Now, one thing I can say for certain is, in many cases guard is not needed at all
. When I first got my Shadow tank to 50 (wayyyy back), I wanted to practice holding aggro. I was moderately geared (few pieces of Columi and a couple pieces of Black Hole). I queued for the group finder and ran a dozen pugs. I never guarded anyone (on purpose).
With using my standard rotation, and taunts as needed, I was able to keep aggro away from the healer. If the dps had
gold fever it would make my life a bit more difficult, but I would keep my area taunt in my pocket just in case.
So. With the fact than in many cases you don't even NEED guard, it truly doesn't matter if you put it on the healer, the dps, or nobody. Heck, use it as a placebo if you want.
This is different from saying it is a GOOD IDEA. Which it isn't in most cases.