Con:
When you require a username to be shared among applications such as web site and email, it can raise security concerns. For example, whoever has access to the usernames in the web site will also have access to the email addresses if the email is use for the username. Usually this is not a problem, but it could be. It is generally a good policy to keep usernames and passwords separate between applications unless there is a common login procedure, or unless security is not important.
