People can still 'play dress-up' while playing anyway even before transmog is a thing (I mean no one's complaining when people eats Deviate Fish, Pygmy Oil, Noggenfogger Elixir (or any other items that modifies your entire appearance)) I gotta admit that I never play BG/Arena to see if this actually works there, I'd guess not in Arena.
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And besides, if you see comments on top, people hardly gets any scroll in raids, or transmog set. And Mercy has said that it's being put to lower priority due to more pressing matters :

