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.
True there is zero drawbacks, and there is zero benefit, because a healer is not going to pull off a target that has been attacked and it does nothing for a group that hasn't been attacked/taunted. Now if I am doing dps too. I will use it, because between healing and doing dps I have pulled from a tank.
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.
No it is not useless for everything, but it is completely useless for adds not attacked or taunted that have already pulled. If a mob is tied to the larger mob but have not been attacked/taunted they are going to the healer, aggro dump and guard is not going to stop it. In some fights putting guard on the healer is actually the way to go, but it is still useless in preventing mobs that have not been attacked or taunted from attacking the healer. Only way to prevent that is for dps to do their job. Of course sometimes it is unavoidable because stunlockers/healers/AoE damagers are more important to kill then the weaks attacking the healer.
It can also depend on the healer. If the healer is new to endgame and not over geared for the instances yet, give them the guard. Still is fairly useless, but it may give them the illusion of feeling safer.
All I can say about my aggro dump, it is great animation on a smuggler. If I feel like I am really in danger, I run to the smashers with gold fever, use disappearing act, wait for the mobs to start hitting the smashers and then start healing again.
That works where the aggro dump doesn't because the smashers will hit everything near them.
ITHE HEALER MUST BE KEPT ALIVE AT ALL COST (this is debatable).
When I first started playing the advice I was given was to heal the tank. Then hit dps with heals as needed and lastly heal myself, but I should never need to heal myself so don't worry about it. This did not work, healer dies the raid dies in content you are not outleveling/outgearing. It was told to me by a tank.
Later I was told healing in progression content is about prioritizing your heals. Knowing who to heal and how much to heal them at any given time. This helps when you have voice, but it is always important to know other classes abilities. It is also easier when you play with a certain players over and over because you learn their play styles and when they use defensive cooldowns and medpacs. The normal priority I use is 1. Tanks, the one on the boss get priority. 2. Other healer, 3 highest output dps, 4 Me, 5 rest of dps, in a pug that will be dps following directions, 6. dps not following directions standing in lava the entire fight. Knowing the highest output dps is difficult at times in a pug, so then I just heal all dps following directions equally. I will not kill my energy management healing the dps standing in lava. I will heal them, but if they are going to cause a wipe by killing my energy, they die.
While healing is my first priority, my second priority is to limit the damage I take. So I move out of AoE and stay away from hit zones.
The last thing I do is dps, I use attacks that don't eat any energy or take very little energy. DPS hard and then the tank takes spike damage and dies because I am out of energy can't happen. Helping with dps is no excuse for letting someone die. Healers job is to keep everyone alive, everyone makes it to enrage on their feet the entire fight, then the wipe isn't the healers fault, they did their job.