Most of it has been covered already, but I'll more clearly state that it's not anything strictly against WHM. Some. people go on about how WHM is "bad" in the current meta, but that's a load of bull unless you're min/maxing to the extreme and actually care enough to restrict an entire class based on a few numbers alone. So don't worry about it.

All of the healers avoid using GCD heals like Cure I and I when playing at a mid-tier or higher Savage raiding level. As has been mentioned, that is because the goal of healing efficiently in this game is to minimize actual healing time and focus on DPS. Since heals are so strong, this is realistic.

Even if you're solo healing, you will spend a lot of time DPSing if you've reached a certain level of healer proficiency.

In dungeons, you're looking at large pulls where you cast Regen/Asylum, the odd Tetra or Bene, Benison, and then Holy spam while rotating PoM/Thin Air and Lucid Dreaming to AoE down the mobs. In older dungeons, this is less the case because mobs hit a lot harder and tanks aren't so tanky.

In any case, rest assured that if you want more intensive healing challenges, you CAN find them. There are things you can do where you will spend almost all of your time healing. However, the downside is that almost never ever does them, since it's old content. And to get that level of healing focus, you'd need to be solo healing it. Things like solo healing MinIL Ultima, or MinIL T9, etc, where the autos do so much damage, you're focused on healing tank most of the time.