Email addresses as Usernames?

One of the biggest challenges of Circlebox (or any website where users need to login) is having an intuitive membership system. For the last couple of years, I’ve been working under the assumption that it’s dumb to have a username other than an email address because the separate username is one more thing to remember when the email address is already a unique identifier.

Tonight I ran across a scenario that has thrown a wrench in that assumption. I’m doing some data cleanup for a client whose website offers online courses and certification tests. Running some queries, I found more than one situation where (I’m guessing) a husband and wife were users on the site and both used the same email address, like this:

John Smith
Anne Smith

They need 2 different accounts on the site since they each have their own certification record. Now I’m wondering if this is the exception or the rule.

I’m interested in any feedback: If 2 people are sharing the same email address, are they going to be OK (most of the time) sharing the same account on a website (that uses the email address for the username)? I don’t think you could have 2 different Google, Facebook, or Twitter accounts that both use the same email address, can you?