There is another reason the war could be good for China. A number of years ago a study came out detailing some of the issues and then possible actions China and India could take due to having too many men. One of those solutions was war.
Below is a recent article from CNN:
http://www.cnn.com/2012/11/14/opinio...e-child-brooks
However I think CaptainCrunch has the best reasoned approach to this. Though I do think that Kim Jung Un will do something so that he doesn't end up looking like a paper tiger.
Is it not possible that the missile system they just moved is an attempt to bait a raid? If a raid happened it could open up more avenues of action for North Korea?