To summarize/add: You not only keep the users email address but you make it easier for users to give you there email address. Check out disqus.com or plaxo.co

To summarize/add: You not only keep the users email address but you make it easier for users to give you there email address. Check out disqus.com or plaxo.com's integration of OpenID, Plaxo in particular gets lots and lots of user contact info for users that use OpenID.