It's a problem when using email to register into a different site. Services without email validation will happily accept those two addresses separately, because they don't adhere to Gmail's internal conventions. If the site then sends mail to its registered accounts, then the Gmail address will receive unwanted mail.
There are horrible people who loves the idea of being able to use someone else's identities. Not just made up but stolen. I - I don't want to know why, nor what they're normally thinking.
