Please read carefully sir:
Let me make this very very clear:
Mitigation barely matters at all in casual content. Someone forgot to Addle in Aglaia? Oh noes! About the only situation where it starts to have value is in dungeon trash pulls, but that's generally more a leveling dungeon thing and even then, mainly if people are under geared. Eating AoEs in casual content? Again, barely matters. I'll merrily sit in some dungeon trash AoEs as long as it's not multiple stacked and continue to AoE through it on my healer. Why? Because outside of key ones such as more dangerous cleaves or the Ochu bad breath, Asylum/Assize will take care of it. They aren't threatening in the slightest. Dungeon bosses even more so, I always get a chuckle when I see people griping about DPS and Tanks eating vulns from Expert roulette bosses. It makes literally no difference to me and if they are doing good damage then all the more power to them.
As for healing being an active ability. That's not entirely true. A WHM worth their salt will be blowing Assize on cooldown in combat as well as burning Lilies if they aren't Holy spamming. Sure, technically they are active abilities, but given that it's frequent healing that's coming regardless of if it's needed or not, it's functioning like a passive heal. WHM isn't alone in this regard.