This is only really true if you’re on Raid healing and you don’t have bored Paladins, and Paladins are almost always bored.
I can snipe 2 FoL on two different people before either of them get a HoT tic. So unless the raid has strict healing assignments, expect to get sniped all day on everything but the most Tank punishing fights. Ironically, the only one that is like that which allows Druids to shine is Vael. Broodlord and Twins both feature very little in the way of raid-wide damage that won’t get sniped.
As for HoTs on a Tank? They don’t really serve a purpose. FoL spam from a single Paladin will put out more HPS and more often, and there are almost no situations where Healers have to move while a Tank eats tons of damage. Even on Twins where you have the potential for Blizzard and Explode Bug, Healers should be spread enough that they all don’t have to move at once. Plus, if you’re stacked to the gills with consumables and wbuffs, HoTs only knock off buffs. I have a /cancelaura macro specifically for Renew/Rejuv/Regrowth and a few other effects when I get stacked.
HT casting, if properly timed, or simply done as a cancel-cast style on the Tank, is really the most effective role for a Druid unless you can manage your Healing team strictly. It sucks, but that’s just how it is.