You say healers are supposed to heal and not dps, but this is a game where you step out of your role to significantly help the team by dpsing, just like a SMN who raises or a PLD who uses stoneskin and also uses clemency which is a HEALING move to heal other players or themselves, it's not a role they were designed for, but they helps healers alot.

Watch this video
https://youtu.be/V06y5hdwrpg

As you can see, the black mage heals as the healer gets KO, is it his job to heal? No its not, but he stepped out of his role helped the party through this dungon. Was his party members upset? No, they were happy that they killed the boss and impressed.

This game is designed to come together as a group of individuals to take down that boss, by stepping out of your role and giving it your all to kill that final boss.

This is a group game, to work together as one.