The problem with using healing as a metric of worth for healers is in this game there is only so much damage being done to the party in any encounter. Fights are extremely scripted.
This means if a boss outputs 1million points of damage over the fight and Scholar heals 1 million points of health it's fine. White Mage can in theory put out, say 2 million. Unfortunately that's an extra million wasted that could have been put into something else.
After a point White Mage heal nukes become superfluous, but the dev team keeps deciding to put their skill points into the pure healing tree when they've already capped it out.