I fully endorse the idea of changing your email address, and using validation emails, but in the past year the following happened to me:

1. My company changed their name (post a merger, and thus changed the address and consequently all our email addresses).
2. I then left that company

Thus in the space of twelve months I've had three email addresses.

Using your recommendation, I'd have to change all of my account names - or just remember my old email address and hope that the site never sent me an email and that I never forget my password.

If I do the latter, I run the risk that someone else gains my previous email address (a new employee with the same name) and they then either

a. Can't use that name for the site because I have yet to change it, or
b. Hit the "Send me my password" button beloved of so many sites and gain access to my account

Using an email address as a username also fails several programming guidelines (i.e. separation of concerns/cohesion).

So personally, I'd never design a site that used an email address as a username, but as it only takes a couple of minutes to add it to ASP.NET as it currently stands, I'd much rather see the ASP.NET team focusing on the bits that are really broken, rather than trivia.

Just as a matter of interest, have you also posted this as a request on Microsoft Connect? It would be interesting to see how many other members of that community want this as a feature.