I get that - but the Google security FAQ suggests that fixing the issue is essentially impossible, whereas I'm pretty sure fixing it is the same as fixing any other CSRF hole.
One reason I've also heard cited is that you always want the logout links in your application to work: you want users to be able to terminate their sessions quickly and easily. If you have a CSRF token tied to your user's session and that user happens to click on an old logout link (maybe they had an old tab open or something), the user won't be logged out of the application.