The problem is that most people won't even click the "Register" link. For me personally it has nothing to do with email though. It's an obstacle which I can't be bothered with, even if it's just username and password.
The advantage of making you register is tracking your progress (and analytics, for the benefit of the webmaster). I suppose it could give you the option to just set a cookie, but that has the disadvantage of being device-specific. Also, some browser crashes can wipe out recently set cookies.