4 healers here, geared to the teeth on my sawbones and I can say without a doubt I am not going to pull threat from tank or dps for my healing. I can also say, threat dump or reduction is not going to help me except in a few situations in ops that have aggro dumps. I was told to use them on NiM TWH for some reason, mine is not to question, mine is just to heal as the group says. Otherwise the my aggro dump is only good for a 1/2 amusing animation. It will not get rid of those attacking me if they have not been taunted or attacked by dps and/or tank. It is just a useless as someone guarding me for aggro reason. Any unattacked/untaunted mob is going to go after the healer. It does not matter if I have 100% present threat or .00000001% threat, if they are untaunted and unattacked by everyone else, then .00000001 is still more than 0.000000000, so reducing it means nothing. More important is for dps/tanks to know kill order and their role. Now I can use disapearing act to drop combat, but the moment I unstealth and pop a heal they are back on me.
Gold Fever a killer of Healers.
As to threat droup by dps, yes that should be part of the roation in group play, especially during/after opening.
There's absolutely no reason that you shouldn't be using your threat drop. It's free, and it isn't on the GCD. In FPs, pulls are typically with multiple mobs. Many times, threat is generated on mobs at the beginning of the fight, and as the fight progresses, your healing aggro surpasses that initial aggro. Your threat drop will either delay that or prevent it altogether. And if it does happen, then it makes it easier for the tank or a dps to get the mob off of you.
There are zero drawbacks to it, and there are benefits (no matter how marginal you think they are). Therefore, you certainly should use your threat drop.
On a similar note, it's not "useless" for a tank to guard you for aggro reasons. It simply is more useful for it to be on a DPS.