Looking at the MailChimp site, I don't understand how that would make much of a difference. Right now, you can enter emails into the "Forgot Username" field and eventually hit a good one, but then you need to crack the email account to get the username so that you can then stick that in the "Forgot Password" form. Eliminating usernames, you click the "Forgot Password" link and enter emails into the field until you hit a good one, giving you both the email and "username" right away.