Omitting healer DPS from damage calculations makes it so dedicated DPS roles can meet required checks. A common complaint with Gordias Savage was it being literally impossible for DPS jobs to perform their role. In theory, this type of design allows the devs to experiment with non-DPS mechanics ala Refurbisher. Nevertheless, it doesn't mean you should be standing still. People fuss over it due to that reason alone. Take Zurvan EX. There are only two instances of heavy damage throughout the entire fight: the second add phase and Tyrfing. If both healers exclusively heal, one or both of you will find portions of the fight where you're waiting or simply doing nothing. Now this obviously does not account for potential mistakes along the way, but you should adjust to the mistakes as they happen or anticipate where someone might mess up. If we don't skip Soar, I'll drop Cleric on the chance people will screw up positions. Otherwise, whenever I off heal, I only drop Cleric to toss up secondary heals (Medica II) or if someone gets a bleed debuff, couldn't get into position for Biting Halberd and etc.
Refusing to DPS is not a play style anymore than a Bard refusing to sing or a Dark Knight keeping Darkside off to converse MP is. The game simply doesn't deal enough damage to warrant consistent healing nor is it reactionary to incentivising waiting for the next "big hit" as it's entirely predictable. Impatience isn't necessarily a factor. Once you start approaching higher level content, the expectation from players increases for everyone. Higher damage output allows for mechanic skips. Without healer DPS, it requires a higher ilvl and near perfect performance from your damage dealers and tanks to skip Lapis in A11S. People want to push those phases because once you've beaten the fight a couple times, that's where the enjoyment comes: seeing what you can skip and optimizing. Something to keep in mind is my emphasis on tanks and damage dealers. They have to perform optimally to skip Lapis. Is it not fair they expect the same from their healers?