It's not about covering people slacking, it's about doing more than the bare minimum. Seriously. This discussion is pointless. It's a simple matter of "why not". Keeping the party alive is extremely easy in most content in this game, and if you just want to do that, fine I guess. But doing that and being able to contribute to DPS while managing your MP is what makes a good healer, in my opinion.
If you've played content in this game where it was necessary to spam heal 100% of the time there's definitely something wrong.
Uh...Is this a joke? >_>