This is in the same vein as RNG mitigation, you assume everything is going to hit. It isn't changing anything about how you interact with the fight. As for crits, depending on the frequency, it might be something you need to worry about, it might not, who knows. But this also has to come with the caveat that getting an unlucky string of crits isn't going to kill a tank as you cannot plan for that as you cannot mitigate everything. It should be noted that this is based on the fact all mechanics are designed to always hit and never crit, if these could crit, that is just a stupid idea and I shouldn't have to explain why.
It's a button to press that might or might not do something, that isn't indicative of skilful play and that is the point as to why it shouldn't be added. It is just a button to press to make it feel like you are doing something, whereas in fact, it has no meaningful impact on the fight. I also find is absurd that you think pressing one extra fluff button for some auto attacks would magically fix the issue of boredom in filler phases. It wwill not.
In terms of mitigation, yes, it is bad. This is something that has been talked about since the start of the game. When Paladin had the original Bulwark, it wasn't used for tank busters as it was unreliable (it was not a guaranteed block), however, it was quite good in trash packs, essentially places where you had more chances for it to proc in the duration. Suzaku's tank buster is a good example of somewhere RNG mitigation could be useful, IF, the mitigation was strong enough when it triggered.
The thing is, we have already been through the era of RNG mitigation, it was generally disliked, the main reason being, you could not predict it. It was not guaranteed mitigation. It is the same way healers do not rely on critical healing when they plan out their healing plans, unless they have a guaranteed way to critical heal, because you can then plan it in.
This goes the same for many other things in the game. Bosses could crit, Paladin had an action that prevented critical hits, wasn't very useful except in 2 places off the top of my head. Shiva EX bow phase as she had a massively increased crit rate that Awareness prevented a lot of damage from the auto attacks and Hallicarnasus tank buster, critical hit. It was an auto crit, awareness is then a form of mitigation by preventing that crit.
Going to Dark Dance, it's main use was in trash packs, Dark Arts Dark Dance + Dark Arts Dark Passenger gave the DRK increased evasion, increased Parry rate and gave the enemies blind, you didn't take much damage, then, coupled with Dark Arts Abyssal Drain to restore health, you could make yourself extremely hard to kill for a duration, though it cost a ton of MP to pull off. (Using guaranteed mitigation with DAAD was my preferred method). However, Dark Dance wasn't used for mit plans, mainly because it was a lot of magic damage, and so it done nothing, but also because, like Bulwark, it was RNG.
The problem is, you cannot do that. Mitigation and healing plans exist for a reason, to make sure you have enough resources through a fight to mitigate everything you need to. You can put RNG mitigation in there, it won't be used for anything serious and it won't be planned around. Even crits might not be unless the crit rate/damage of the boss is significant enough.
But this is still a spreadsheet. Everything will be planned around, those randomised party hits are going to be done at the same time each fight, so you already have the resources allocated to deal with it, it is just you hit player 2 rather than player 1.
Many tanks do enjoy planning out the mitigation strategy, that is part of the joy of tanking. I am mitigating as much damage as I can whilst staying alive. This is done in tandem with healers and planning out their healing strategy. The healers might have a better way to deal with something, they communicate that to the tank, the tank can adjust the mit plan etc. And I should note, nothing has changed in this aspect since ARR. This has always been the case.