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I believe that the PRC is using NK as its personal experiment station. Yes, NK is used to proliferate weapons and policies that would otherwise get China in trouble. However, the CCP can use NK as a guinea pig to test the US (and World's) reactions to a variety of scenarios (such as pulling out of non-proliferation acts, developement of nuke threats...etc).
Nevertheless, the CCP will start to sweat if they lose some control over NK and Kim starts making aggressive moves towards his neighbors triggering potential backlashes by Japan, SK, others as those backlashes could lead to closer US (particluarly military) relations. It could be a disaster for Beijing if Japan feels threatened enough to boost military spending and, worse, begin development of nuclear programs.
As a testing gound, the CCP can hone its political skills (especially while avoiding recourse by having NK's nutty leaders blamed for whacko decisions) and, as stated, formulate better tactics to establish their anti-west panacea (thereby creating a significant buffer against "American thingking"). If they succeed they would also create a significan stategic zone of influence. Of course, this is contingent on IF they can control NK's leadership and IF the people of NK don't revolt (hard for the starving to do?).
One last caveat is that the PRC could actaully benefit from NK's collapse. With a united Korea, and the current SK anti-american wave, why not tell the US troops to go home. So much for US influence on the border and the CCP would find it easier to project its 'soft power' into SK. If the CCP can effectively control the collapse (i.e. NK doesn't start invading the South or shooting nukes when the collapse occurs), it could certainly work out in their best interest.