You can do that with a username....

True however then users have to think followed by get frustrated dreaming up a name. They already know their email address. And unless they already signed up its probably not taken.

I think the frustration of entering your email, checking you inbox and following a confirmation link is a lot more than the frustration of dreaming up a name - which you have to do anyway unless you use your email as a login.

I think that the metrics support this view as well.