When google.cn was available but censored, anyone in the US could simply run the same query on both and make a list of the missing results in google.cn to tell what was being censored.
Such a list lent itself to easily gathering the censored documents and distributing them via other secure means to China.
Now, thanks to Google, that is impossible. Did this fact really escape the minds at Google?
Google.cn was a great window into what the Chinese government doesn't want the Chinese people to see... now it's gone.