Dear Healers who don't DPS

[Taika]

Again, why must a healer DPS? There are compelling reasons on why they should DPS

You're really answering to your own question here. They should DPS because, if, and when they can. If you can be helpful to your party, you should, and it's fair for others to expect that from you. If you can be helpful to your party, there's no sensible reason or justification for choosing not to.