For exactly the reasons Jeff Jarvis illustrates right here:
Listen, I am the first one to say that people should be accountable for their actions and that anonymity isnt a license to act like an asshat. But our founding fathers knew that anonymity was the people's main defense against tyranny and I'll be damned if I am any part of a system that has as it's basest core a lock out against it. I'd argue that its as or more important to maintain the ability to remain anonymous online as it is in meat space. Yes it has incredibly far reaching consequences when anonymity and security meet. When those collide, there are sufficient laws for dealing with them already.