Healers should be DPSing to avoid falling asleep out of boredom on everything but the most healing-intensive Operations boss fights after about level 30. Bored healers lead to wipes.
HPS requirements for nearly all content are ludicrously easy to hit, even at content-appropriate gear levels, so an efficient healer is going to have a lot of downtime. If they don't want to waste resource regeneration, they might as well DPS with the extra resources that would do nothing otherwise.
Corruption Healer here....
Dear Tanks, one Recklessness --> Dark Infusion = 50% HP
I am bored, so I am throwing out Lightning.