I come across this question a lot. Although you could argue that you will be going back to China years later, I still don't see why you should not contribute to 401(k).
I believe the people who ask this question have misconception about the purpose of 401(k). The purpose is to encourage you to invest for your retirement. Let me be clearer, investing does not mean a few years, so the purpose is not to let you withdraw the money when you are still capable of earning money when you are young or middle-aged, whether in China or the US, barring some very unusual events, like huge medical expenses, purchasing home, or you want to start a business of your own. Unless you have to, you should leave it in your account.
The benefit of a 401(k), you didn't have to pay some of the taxes, and the advantage of letting it grow in your account over a 20-30 year span TAX DEFERRED is humongous, there are lots of examples on the web that can show you how amazing it is. see www.fool.com for an example.
Plus, lots of companies match your contribution to certain percentage of your salary, which is free money, so why not take it?
And why do you have to take out the money should you leave the US? You can roll it over to an IRA account. I know Vanguard doesn't charge you anything should your overall account value exceeds $5k, and only $10 if <5k.