The plain unvarnished truth of the matter is that WoW is the 800 pound gorilla of MMOs and as such, if the numbers of 6-8 million subscribers are true... in fact if they are anywhere close to being true, then you can bet your lungs that more people have played WoW than all the other MMOs put together. And because of that, many of the ones who drift away and start playing other games are likely to point out where they've come from.
Think of it as a kind of cyber-street-cred!
Personally I agree with you... I don't care where someone has come from or what they have or haven't done before. All I'm interested in is what they're doing and what they know within the confines of the current world.
But that reminds me, did I mention that when I played WoW I... :)