For obvious reasons, this doesn't apply to this site.
But seriously, part of one's security is also having a hidden username. E-mail addresses are designed to be shared information. You basically lose half of your basic account protection unless you go out of your way to make a new e-mail for each site you use. I can kind of understand the convenience angle but what the hell is the deal with forcing it? Are they going to tell me to use phone numbers as passwords next?