Don't forget gear gaps in PuGs... we have played agro games with a friend... I used my 66/69s (66s main/off hands and some drop tank 69s) geared Shadow tank and he was on full 78 geared Sage DPS.
There is no way tank can keep agro with such cap (shadow rotation is not so complicated on stationary targets after all) without taunting... and there are some fights when taunt is not good to be used in wrong time.
That was with a geared DPS, and I'm not arguing that a geared DPS cannot pull threat from an undergeared tank. We are talking about pulling threat as a healer and needing to use threat drops, and I maintain that it just isn't a significant skill for a healer at endgame. There is quite literally only a single instance where I use a threat drop when I'm on my healer, and that's on TfB second phase. The common cases are:
1. You are doing story content where the incoming damage is trivial. You are not putting out 7K effective HPS, you will probably not even break 3K eHPS on average, unless you are solo healing. Even if your tank is in DPS gear with no mitigation, your eHPS are never going to be high enough to pull aggro from a boss. And note that it is effective HPS that matters - overhealing does not generate threat. Against trash pulls or adds, we already know that threat drops and guard do nothing for a healer's threat.
2. You are doing progession content that actually stresses your healing ability, the tank's mitigation, and the awareness of your DPS. You are not doing this content with a seriously undergeared tank. If your tank has the gear to survive the hits in the first place, they have the gear to hold threat off a healer. Probably not the most geared FOTM DPS, but that is what guards, taunts, and tank swaps are for.