A RhetNet SnapShot Reply:
Even though I'm a little more tentative than you seem to be about completely abandoning traditional essays (even if I want to move beyond traditional essays, professors in other departments haven't and that is a very large reality to me) but I wanted to add one thing to the big advantages of having students do more collaborative and evolving interactive writing.
The big advantage I see is that my students learn to negotiate differences and compromise when they are on the web. That is something they never seem to get much practice in when they sit alone and the emphasis is put on "take a position and defend it to the death." When they do interactive writing, they are in a situation where they cannot escape other's views (though some still do ignore them), but they are always exposed to other thoughts and in a setting where it is more difficult to dismiss opposing opinions as the "type of thing THOSE people would think." It's a type of learning, writing, and thinking I wish more members of our government had practice with.