Why would you, for a local authentication site, have usernames, instead of using e-mail address to login?

In websites where authentication is local, or mixed local and remote (e.g. OpenID), I still often see the question for a username. I can very well understand this in the case of twitter, where the username is used in many ways. But, in many other websites, like a webshop, the username is just for authentication.

So, is there a good reason not to just authenticate on e-mail address? Are usernames mostly obsolete?

Answer

Usernames provide a "face" to the world. They're great on forums, etc. to hide the identity of the poster, or allow some personalization. However, for the purpose of logging in, e-mail seems universally better. Most sites I've seen with usernames have a "forgot username" button right next to the "forgot password" button, which should tell you why :-).

For a worst case scenario of this, Washington Mutual's website used to have login via e-mail address. After they were acquired by chase, I was automatically assigned this wonderful username: fraserofthenightgmailcomwmb2715 (with different numbers -- but they're totally random, not related to my account number). Guess how many times I clicked "forgot username" until I realized I could write it down...