If you're aware it's optimal and situation dependent, I am not sure why you're exactly complaining about the concept of healer DPS unless you want to put in less effort to get content done.
And no, I'm not saying it's necessary. I don't believe I've ever said it is or was. I did say that it was part of the meta and the meta is defined by something that started with a small group of people and has been adopted by a large group of people. It takes the majority for a concept to be called a meta. You seem to experience this kind of democracy as a "dictatorship". Which is something I have also mentioned in another topic where I replied to you. So I'll break it down to the root of you and hopefully you'll understand why the healer is pushed with extra duties aside from just pressing that cure button.
Healers are simply in a position to make an encounter more forgiving for everyone. Tanks can only take so much damage, DPS can only deal so much damage. They're gated by gear, mechanics and player skill and are often challenged by the encounter on these two things: eHP and DPS checks. Healers can offer more mitigation to increase survivability. Meaning less HP required before attacks can be survived on their own. Tanks (and dps/healers as well) won't instantly get punished for not being overgeared or for entering at minimum ilvl requirement. Or in other words: Mitigation from healers make an encounter more forgiving.
Remember when I mentioned DPS checks just now? Healers are also in a position to help ease this up or "make it more forgiving". DPS jobs (and tanks) are limited to their gear, skill, and encounter mechanics. Because healer damage isn't included for DPS checks, it eases up said checks. Or rather: It makes DPS checks more forgiving. It allows the group as whole to make mistakes in their rotation and suffer potential damage loss. It makes lack of gear more forgiving as well.
The healing output any of the healer can put out simply blows away any form of HPS checks the game has right now. Sure, there are times when both healers need to heal. But there are also times when neither healers have much to heal. Until Square-Enix introduces content where the healers are using every GCD on a healing spell and are rooted on the ground because they can't afford to stop casting, healers will always have time to make an encounter more forgiving for the whole party. So why bring two healers instead of one if the HPS checks are so low? Because that'll make the encounter more forgiving for those in the healing role! On your own you can afford no mistakes. Together it makes mistakes more forgiving. If you simply don't like making encounters more forgiving for others, others may not be so forgiving for not putting in that extra effort. And concerning effort, I believe we have thoroughly discussed this in another topic you've started.