You've managed to make a bunch of ubsubstantiated claims that read more like an opinion piece on why standards in multiplayer games are bad than anything remotely akin to an answer to my question.

Why shouldn't a healer dps when they have the time and mp to do so?

There are virtually no MP limitations at high-level casual content. I've raised 25+ times in Rabanastre while dpsing as a WHM and didn't run into MP issues. There are certain times throughout the leveling process that dpsing too hard could get you into trouble, particularly when you first get Holy as a WHM, but overall there is no real MP constraints healers face if they have even a modicum of skill. This excludes extreme circumstances of horrible parties of course, I'm not advocating for a healer to dps when it would be detrimental to their group. This is about helping.

In terms of time constraints... Let's be real, virtually none exist anywhere in casual content. To have a constraint on your time you would need a high enough CPM to be hitting that kind of cap and heal-only healers are virtually never there. If they are it's an egregious amount of overheal.

How about you try to answer the question that is being asked instead of creating a new question to answer?