If boss fights were actually designed with the sub-100%-uptime AM model in mind, then sure, but they aren’t. Bosses hit hard the entire time, and their “hit really hard” abilities are timed and tuned for actual cooldowns to be used against them, not just AM.
If bosses had fairly regular but properly spaced and telegraphed high-damage windows/events, that were both timed and tuned for AM mitigation, and their “normal” DPS in between could be handled without AM, then sub-100% uptime would work just fine. But that’s not how it works right now. They added AM, but the encounter designers never seemed to take it into account.
I didn’t raid at anything approaching a competitive level over the last two tiers, but back in BoD, Jaina just continuously stacked her debuff, Mekkatorque just blasted the tanks, Bwom on Rastakhan was pure magic damage, Paku just continuously stacked his damage buff, Gonk’s bleed ignored armor (ie. most AM) and also occurred too often to effectively mitigate with AM even it was possible, Kimbul’s was also a bleed, etc.
Like, none of these fights were designed for tanks to use AM as a counter to abilities or buffs or debuffs or whatever. They were designed to just hammer the tanks, and AM simply contributed to overall average mitigation. Sub-100% uptime simple doesn’t work in that model, because it results in you being tankier than you really need to be while AM is up, and simply going squish when it isn’t.