Originally Posted by
Thalur
To turn the question around: why do you want to cast a direct heal? Of course, if someone needs more healing (from you) than your Rejuvs on him can do. Why does he need the healing? Because otherwise he might die. Is that a situation where you start casting a 2.1 sec HT that will do some mediocre healing? No. If someone needs healing now, you use Regrowth or Swiftmend. If you use HT, either he dies or another healer provided the healing needed. (Or the healing wasn't that important after all.)
There are situations when it's useful to cast HT, but they are rather rare and many resto druids are over-using HT, so I try to generally advocate not using it. It is often a good idea to cast HT on the tank (with Lb) during high tank damage phases, here HT can be seen as kind of pro-active healing (as you have no idea if the healing will be needed when the cast is finished, but better be safe). Casting HT on a non-tank is usually the wrong choice, as mentioned above - either HT is too slow, or Rejuv can do the job better for less mana.