I distinctly remember Blizzard stating that every spec should be attractive and that they NEVER want 1 spec to be 100% dominating one.
Well, check 3v3 ladders (as 2v2 is no longer deemed 'competitive' by blizzard) and filter by class. From the first 50 results see how many warlocks are NOT affliction, warriors are not arms and druids are not resto. It goes for all classes.
Oh well, I guess it goes 1 spec for pvp, 2 specs for PVE. WoW has always been a PvE game. Ofc no one forbids you to play fury in arena it's just that the spec it completely not suited for that and although you may get pretty high in the rankings, you will struggle significantly more than others with the 'proper' spec