People play also because they have to get their money's worth. I'm quite invested in GW2 when it comes to my guild, the friends I've made, but no I don't feel I 'have' to play the game, so I've gone a week without logging in at times, generally when there's a big new game out or something.
Gotta say it's nice to see Anet releasing new content on a monthly basis. It's already seen more new content than some P2P MMOGs see in a years time, sans expansions. I'm particularly looking forward to this Feb/March mega-patch.