A party can get by with the healer just healing in leveling dungeons. It'll cost a couple of minutes off the run, no big deal. Yes it'll be a pretty bad healer, but they exist and help reduce queue times. and the world doesn't end. Same as a group survives with a dps that does very low dps, or the tank do low dps. I doubt any leveling dungeon group has seen any drama from that, and if so it's easy to report as we're in ACT land.
When someone insists the same goes for higher end content, and even advocates that as the norm, it's a different story. Expect healers to protest an extremly monotonous experience.
Healing wise, I think they've designed themselves into a corner. You want healers to be able to contribute to the group in a way that continuously rewards further improved player skill. It's not intuitive how that can be done for the healing itself, with a design based on predictable intermittent damage, gameplay heavy on cooldowns, and generally small mana pools with high regen. So healers don't contribute to clear times except by their ability to dps. And for reasons unknown, that dps is designed to be incredibly brainlessly monotonous. That, at least, could be fixed.
I wonder how it would be if the healing was gcd and the dps was weaving in ogcd more or less optimally depending on skill and knowledge, but suspect that wouldn't help much with the mana the way it is.
