I am sure I will not shock anyone by saying that an email address by itself is not good enough for authentication. It is to easy to fabricate an email address, to create a one time one, and to use someones else address, so why exactly do we still use it as an authentication token in wordpress comments?
It is not that getting the email of a commenter is a bad idea, it is just that is not enough for authentication. What is needed is a way to proved that said email actually belong to that person. One idea is to send a mail to the email address and ask to confirm the submission of the comment. After verifying the email it will make more sense to get profile data from gravatar with this email address.
And there is a different approach that avoids using emails for authentication – use the commentator’s profile on the web. Most of the commentators have a facebook/google/twitter/tumbler/wordpress.com/flickr account with a profile, just let them authenticate their profiles. You can even get an avatar image and maybe name that you can use to identify them to the readers when displaying the comment.
This does not necessarily work against anonymity but you probably be more inclined to approve an authenticated comment then one which is practically anonymous.