So how many buttons do you want to press?
Is 2 enough? Or does it have to be 3?
1-2-3
What happens if you need to break combo? Or even if other skills don't break combo, what about burst windows or optimising the 3?
Healers can absolutely not having anything that punishes your dps output for interrupting or even delaying a combo, for even a single GCD.
There's already backlash for the fact that GCD heals don't completely reimburse your dps, how bad would it be if there were damage ramping from combos to take into account as well?
"Oh you can't delay your Holy 5 to cast cure because that's a 300 potency loss, you can only heal after a Holy 3"
Plus where are these extra skills going? Button bloat is already bad enough on healers, so you'd have to lose healing skills to make room for them. Then you're dumbing down the actual healing of the job too.