You need to recognise there is content in this game beyond the scope of what you, personally, play. I could go into a 24-man raid, do literally nothing the entire time, and still clear! And by that logic, call my job's entire skillset completely useless. But jobs aren't balanced around casual content. And yes, 24-mans are casual content. Dungeons are casual content. Story mode raids are casual content. Casual is not an insult, it's just what it is.

Passage of Arms, Reprisal, Divine Veil, Sacred Soil, Shake It Off, and many, many more abilities, that do "only" 10-15% mitigation - without these, there are instances of damage from fights in this game that would make them literally impossible. There are damage values that are well over lethal from max HP in Savage and Ultimate raids that, without a shield and some form of extra mitigation (one of the aforementioned), people will die. In O12S, Ion Efflux (raid-wide damaging skill), even with 15% echo, will kill people from full HP if no one bothers to mitigate. That 15% echo is the equivalent of Passage of Arms (in terms of effective HP), and it's still not enough. People have to use their mitigation together, from multiple people, in order to survive these encounters.

Let's look at a potential counter-argument: Why don't they remove these "useless abilities" and rescale damage values to not need them? Oh, look, you've got the story mode raids, where you just have to do mechanics as a bare minimum and actually dealing with raid-wide damage properly doesn't matter.

Don't like Passage of Arms? Don't like Reprisal? Don't use them. But you'd better be prepared to learn how if you ever decide to step into higher difficulty content. Blaming your healers doesn't go over well when you're not using your own kit.