Healer & Tank DPS (WoW vs FFXIV)

[zipzo]

If a healer is casting nothing, there is no major damage incoming, and the healer isn't out of mana, why wouldn't they deal some damage?

So, my first instinctual healer-based reflex response to this question I think is a rather obvious one...but what if things don't go perfectly smoothly and you need that extra mana later to fix a catastrophic healing demand?

That's mana you expended for damage earlier that you could have had in the chamber in order to execute your actual role to fix a tight situation. So wouldn't this logic only accurately apply in a world where the healer has infinite mana?