When you have to create a username on another site to play a game on this site, you're going to have some attrition. Especially if someone you don't know, even in the vaguest sense of "internet know" sends a sitemail saying "Hey, send me your email. I need to send you an invite to this other site." Dude might pound my email addy with porn site invites.
Well, more than I already get.