Tom Barnett knows why not to boycott the Games:

My prediction on Beijing 2008 (Thomas P.M. Barnett :: Weblog):
Here's my prediction: Beijing puts on a great Games and stuns the world by winning the most gold (although U.S. wins overall count), but cannot escape a profound sense of shame re: how much they've trashed their environment because tons of news stories generated at the Games focus on that, to Beijing's horror.

As a result, the Games trigger intense debate and further national focus on stemming/reversing the profound environmental costs associated with China's rapid rise.