I believe the reason why Arms didn’t receive those damage buffs in PvP is because Arms damage in PvP is relatively good.
Where Arms suffers, or I should say where Warriors suffer in general, is their ability to survive. If anything, I’d argue that this isn’t necessarily a bad thing if more classes were just as vulnerable. Of course it doesn’t help that one of our best survival tools (Impending Victory) as of right now is a talent choice that needs to be picked over taking our biggest form of cc (Storm Bolt) or more mobility (Double Time).
I would argue that if anything, more classes should be toned down bit so they don’t have the ability to potentially “one-shot” nor should they be so difficult to kill by comparison. By un-nerfing Defensive Stance, it would only promote the latter as Defensive Stance prior to the nerf, albeit previously balanced versus being a current net loss, coupled with Ignore Pain and less than 1.5 minute cooldown on Die By the Sword, had Arms Warriors less likely to be a focus target due to being hard to kill.
The reason why Fury is excelling at the moment is because they do comparable damage to that of Arms Warriors, if not more when they pop all their cooldowns, whilst also providing a consistent pseudo form of CC, a 40% healing reduction. This grants Fury more freedom to pick other talents such as Impending Victory or Double Time based upon their playstyle or match-up instead of feeling as if Storm Bolt is always the best option. Couple this with also having their “Defensive Stance” like talent (Warpaint) to have no drawbacks other than it might not having a 100% uptime if you’re not continuously attacking, to which I might add that Fury has more mobility options at their disposal when compared to Arms Warriors (the biggest being Bloodrage). And let’s not forget that Fury has more self-healing over Arms, a lot more.