Shield/Pure Healer Split Feedback

[Rilifane]

And in the times it's NOT optimal, it won't matter, since no fight will be decided by one 295 potency cast being missed.

The question of whether something is necessary or not is and never will be part of discussions of optimal gameplay specifically because optimal gameplay aims to go above and beyond of what is necessary to clear by nature. That won't change unless SE designs a fight which requires optimal gameplay which they understandably won't - that would set every social media on fire, they already came close to it with Midas.
As such, optimal gameplay includes and requires that you will clear but does not care about if it was necessary to clear or not. It is an approach that aims at maximizing personal and group improvement, not leaving it at clearing alone.
So the argument "But it wasn't necessary anyway and if it was, your group has other problems" is completely besides the point.

Optimal gameplay can be seen from a personal and a group point of view and while they overlap most of the time, it's not the same.
Any increase in pDPS will automatically also increase the overall rDPS but it's generally not considered optimizing group play unless the rDPS gain surpasses the pDPS gain. An example of this would be a BLM using Transpose lines outside burst windows. Nobody else but the BLM benefits from this at this time and as such, it's simply personal optimization.
A BLM banking Xenos to dump them during burst windows on the other hand is more part of group optimization because this specifically targets maximizing other people's buffs. Personal vs group optimization isn't split into "selfish gameplay that hurts others" vs "good gameplay that benefits others".

So yes, optimizing gameplay can definitely mean using a GCD heal - if it was necessary to survive and/or saved further dps losses down the line. Every GCD heal that does not fall into this category is not optimal and it doesn't matter if the additional Glareoileficosis would've made a difference or not for getting the clear.
You do understand what optimal gameplay means but you bring something into the discussion that has no place in it: would it make a difference?
Doesn't matter.
A BLM using Transpose lines won't make a difference.
An AST catching burst phases with cards won't make a difference.
A greedy melee getting one more GCD won't make a difference.
Not even all of it combined would. Not even at min ilvl in current savage.

The framework of what is optimal is set by SE and whether we like it or not is another question that doesn't matter in these discussions. Doesn't mean it shouldn't be brought up. Just not when number crunching.
I personally don't like that optimizing healer means doing less instead of more when it means doing more for every other class. But the framework SE gave us is that dps is king and they keep enforcing it by switching from vuln ups to damage downs (which made a GCD heal or two from healers optimal if it meant the whole party could cheese a mechanic), by avoiding as much controlled dps mechanics as possible, by handing out more and more mitigation/ heal tools to non-healers, by keeping incoming damage the same at best and by keeping everything perfectly predictable and scripted.
But it's still optimal.