WoW has historically worked on a daily limit to progression model for the endgame, so the 3 day early access is potentially a 3 day permanent boost for the people who buy it. I would imagine competitive raiders going for world first and "clearing hard difficulty versions of raids while they're current content" achievements and their related rewards will be essentially mandated to buy it.

As for gamers obsessing over things at launch, I wish it were different, but I think of it like movies or TV shows. If you go and watch a movie a year after it came out, nobody is gonna be talking about it anymore. And for some people, that social buzz around a new piece of media is half the fun. Playing a game and talking about it with your friends, the sense of discovery finding things out before you can just look it up on some wiki site, etc.