MVC 2 AntiForgeryToken - Why symmetric encryption + IPrinciple?

It was added to offer greater protection in the case where you have one subdomain trying to attack another - bad.example.com trying to attack good.example.com. Adding the username makes it more difficult for bad.example.com to contact good.example.com behind the scenes and try to get it to generate a token on your behalf.

Going forward, it's possible that the cookie will be removed as it's not strictly necessary for the proper functioning of the system. (For example, if you're using Forms Authentication, that cookie could serve as the anti-XSRF cookie instead of requiring the system to generate a second cookie.) The cookie might only be issued in the case of anonymous users, for example.

Besides the "evil subdomain"-scenario outlined by Levi, consider an attacker that has an account on the targeted site. If the CSRF-token does not encode user-specific information, the server can not verify that the token has been generated exclusively for the logged-in user. The attacker could then use one of his own legitimately acquired CSRF-tokens when building a forged request.

That being said, anonymous tokens are during certain circumstances accepted by ASP.NET MVC. See Why does ValidateAntiForgeryTokenAttribute allow anonymous tokens?