I agree in that it feels like a general over-arcing insult to WoW and the player. For the short time I played Aion, I saw that phrase used all the time in game, and I still see it on occasion when I play Warhammer online right now. At least in the Warhammer community, more people than not also see that problem, and many people will actually call out people who say that, pointing out how stupid the retort was.
I have also seen "Go back to WoW" used in proper context too, which makes me kind of sad. People will show up in game, complain endlessly about the class, and it will basically amount to "Like a rogue in WoW?" While this can be a bit common as well, in the end, I still see everyone as at fault. Input is input, but many people also do just come to make that 'MMO x to WoW' comparison.