The reason I included E-mail Addresses (Of limited use.) is:

• Joe signs up using JoeUser as a username and joeuser@isp.com for an e-mail address.
• Joe decides he wants another "persona" and a new User Name.
• Joe decides to use JoeBlow as a username and joeuser@isp.com as the e-mail address.
• Joe now has two accounts; which could potentially be used to vote twice. If a query is run across the database finding user names associated with the e-mail address joeuser@isp.com (in this case: JoeUser and JoeBlow), it could be coded to only accept one vote per e-mail address required during registration.

Hope that makes it clearer. ;0)