The email address is a great way to uniquely identify users. Also storing the email allows a website's authentication system not care about what API the user is using to login into their site, wheather it be twitter,facebook, or OpenID. Also, having the email would keep you from incorrectly creating a new account for a user for your site if they happen to use another method of logging in beside twitter. An example of a site that does this is Stack Overflow. It doesn't care how you login, it still recognizes you as being you.