Healers can do ~10k + dps, with the right gear/conditions. Sure in an overwhelming majority of keys 15 or lower their damage won’t matter, but why not do it? Maybe a newer healers damage will be next to nothing, but it’s a start. Tossing out a few dots between heals, building comfort with that and moving into more damage spells as they get more comfortable with the playstyle. Then if they start doing more challenging content they will already have that comfort with doing damage, and if they continue to do 15’s only they’ll at least blast while doing it.
I’ve seen healers all over the spectrum of damage done throughout keys and the only ones I don’t want to continue to run with based on damage done are those that don’t use a single damage spell. Just seems lazy.