An off tank can rest (afk) while there are no adds to tank or tank swaps to be made?
The aesthetic/title ends up effecting how that character actually works, so yes. It's always been that for Final Fantasy. I think the issue I personally have with the opposing side is that when it comes to, as you said, "how the game should be played", that is very subjective with varying degrees depending on what content you are actually doing and how certain jobs are actually designed.
The issue boils down to not what is most efficient, but forcing a view of something optional as mandatory because of the potency, ignoring what that person is actually capable of, ignoring that not everyone wants to play as cutthroat as this just to win a 4 man dungeon run a few minutes faster. If you think it's silly to use "cutthroat" for Healer DPS in this context, or anyone mind you, for a lot of people it actually is very difficult to manage healing and DPS with the Cleric Stance mechanic. There's a reason Healer is regarded as the most difficult in this game with meme images, it's because when you tap on DPS, the challenge sky rockets.
Personally, I always DPS as WHM as I can, and I enjoy it. I don't feel it's anything that should be forced unless you are deliberately setting up a Party Finder for people going efficient as possible on farms or raiding.
Except they don't? Games are games because they have rules and mechanics. Rules and mechanics (read: abilities) dictate the gameplay. Labels and descriptions may build immersion, but ultimately have no impact on gameplay.
That's like playing a match-3 game and refusing to get higher score by matching 4-6 elements even if the mechanic exists, just because the game is called a match-3 game.
This whole discussion can be summarised by contribution. A healer standing still is not contributing to their team whether it's an 8 man raid or 4 man dungeon.
Yes, I think our points of view simply differ here: I do think the way the jobs are supposed to be played is determined by the abilities and the battle design and what is most efficient way to play them and it's not subjective at all. Furthermore, I don't understand why people are willing to accept it to be subjective for only healers, but not for any other job.
I don't personally agree healer is the most difficult role to play in the game, having all jobs but MNK at 60 and having done raid content on both healers and DDs, but I think that's truly a matter of opinion.
I think your misinterpreting my argument a bit. Saying "healers should DPS when possible", doesn't mean they should push themselves to and beyond their limits on every single casual dungeon run. If that were the case, I would agree that it's an unfair demand. For me at least, the only thing I actually ask is that healers would try to actively and effectively contribute in their team. If everything is going smoothly and no one is taking nearly any damage at all, then yes, I expect the healer to DPS, even if just a little bit, and I don't think that's too much to ask. However, if healer is struggling for any reason related to themself or their party members, and have to actually work to keep up with just the healing requirements, then it's perfectly alright to do just that. What I dislike is when a healer is perfectly comfortable with their job and the content they're doing, and only have to be active for 20% or less of the run to cover the healing, and then they simply refuse to do anything else to help their team, when they could. In those situation I definitely think the healer can and should be required to DPS instead of being afk or intentionally spamming cure or stoneskin when neither is needed.
It doesn't cause issues within my static and most other groups i know also play like this. If it doesn't cause issues then whats the big deal with healers not doing DPS? Honestly i fully agree with YoshiP. The role is called Healer, you should be Healing as your main focus. If you WANT to DPS then go for it but its in no way a REQUIREMENT to clear content.
I wouldn't say no impact on the game mechanics per se. Like as stated previously, because Bard the title is a musician, they play music in-game. Monks generally fight with their body and that in turn impacts Monk being close range in this game. I'm not saying this is everything, mind you. The titles of the actual Roles are a bigger deal.
MNKs don't fight in close quarters because they're MNKs. They fight in melee range because their abilities dictate they must. They're called MNKs because it fits the lore of the universe and aesthetic of the class. Not every class that fights close to the boss is called a MNK afterall. BRDs also don't play music, they use AoE buffs that have varying effects, song is just a label, animation and aesthetic. Buff/Debuff is the mechanic, song is the aesthetic.
Party roles also don't dictate gameplay at all, they dictate party setup, that's literally their only actual function. Changing the role labels from "tank, healer and dps" into "wall, cross and stick" or "blue, green and red" would have zero impact on gameplay, only game feel and immersion.