Some people use many email addresses and so could create many accounts. With one email address they can still use the '+blahblah' method to sign up unlimited times, unless you prevent that which would annoy people who use it legitimately for filtering.
Some people have a garbage or throwaway email account that they sign up for everything with, and only ever look at to find the confirmation emails.
If people don't want to give you a valid email then there's no reason to be sending them anything.
