Threat drop is at least for me who am still leveling FP healer, useful tool, when there comes too many mobs on me to manage. It can save my life.
BUT with PUGs it has also caused one votekick and some annoyed responses from DPS/tank so I use it as little as possible
Those tank and DPSs was surely crying for place in your ignore list.
On FP ... if mobs are on you drop may only help tank/dps get them faster ... if nobody attack/attacked those mobs, drop will do nothing.
When you go to multiple healers content, drop most probably will send those mobs to next healer... if not attacked again.