The problem with healers in MMORPGs is always the same:
- In order for you to feel like you're needed and your job is challenging the party has to take enough damage that they would die if you weren't there.
- In order for you to succeed in your role the party has to not die from all that damage.
- Since no content can be completed unless the healer succeeds in their role people simply expect the healer to negate all regular damage.
- Once regular damage never kills anyone there need to be mechanics that kill people even when they do get healed.
- Once every fight contains mechanics that can't be healed through those become the de facto challenge to stay alive in the game.

Basically healers are victims of their own success. In order for healers to be needed people need to die without them. In order for healers to succeed once they are needed people need to never die. The game has to invent new mechanics just to create danger that healers can't negate. Those mechanics become the real challenge of the game. Healers become kind of irrelevant simply because they were so good at their job that they had to be made irrelevant to still have any challenge.

One thing they could do to make actually casting healing spells a little more attractive would be adding something like an overheal bar to people, where if you heal them when they are at full health they become powered up. That extra power can be burned off to make attacks stronger, so a healer can indirectly contribute to DPS by keeping the DPS characters topped off with overheal to burn rather than having to be a pseudo DPS themselves.