What are the pros and cons of using an email address as a user id?

Question

I\'m creating a web app that requires registration/authentication, and I\'m considering using an email address as the sole user id. Here are what I see as the pros and cons (upd

Answer 1

CON: If a hacker can try registering random email addresses en masse, he or she will be able to figure out which of those addresses are valid based on which registrations fail. This is a tactic that can be used to put together lists of known valid email addresses, which are a hot commodity on the spam black market.

Although now that I think about it, that's a problem that affects any website which asks for an email address as part of the registration process, regardless of whether or not there's a separate username. But it's still something to think about.