I'm not sure.
I think it's an inevitable policy and is a part of a greater strategic plan. To date, China has been very effective at gaining kudos and brownie points from the international community through it's DPRK strategy - you really don't hear much from the west about how China should be doing much more. There could be
far greater public noise made over the issue - it's not happening. Thus, it seems to be a fairly succesful strategy at the moment. And the whole six-party talks stuff has presented China in a very positive light.
Better to have a buffer than a war, surely?
Yes. This has been the status quo for many, many years now.
Ultimately, it's unlikely that the DPRK itself will unleash chaos. Beijing has enormous influenf over the country. The survival of the DPRK depends entirely upon China's patronage.
Well, China is the worlds fourth largest economy and the Central govt. has nearly a TRILLION US$ in cash in the bank. A few hundred million every now and again to "own" the DPRK is small change.
The US (as usual) is pushing things at the moment and there must be underlying tensions between the US and China because of this. That said, the restraint on the part of the US in any public criticism of China regarding it's role in game suggests that China must certainly hold a pretty strong hand. I believe it does.
Woof