I can see that your trend data is probably incredibly accurate, but I guess the down side of ONLY recording data from those allowing cookies goes back to accurately defining how many people are on your site not using cookies. I think that you are probably right not to pay any attention to these people at all, but I still feel that blue-sky and long term that will be a problem. Especially in Europe (see the new legislation in force at out-law - [out-law.com...]
Also, how many pages get half loaded and people move on before the cookie gets properly recorded.
I guess it is the law and paranoia that is going to limit cookies in the future, not technology.