As already pointed out, the
Libyan scenario is what they fear here.
As
kaiserd mentioned above - for economic aid. In fact they have already done so in 1994 when they committed to freezing its plutonium weapons program in exchange for aid. Basically despite all the bluster and internal cult worshipping, their leaders know that the country is really a basket case and if it were not for their
potential to cause grief to Japan or Sth Korea, they would be ignored totally as a backwater. This is even more of an issue since the end of the Cold War in the 1990s since the US and others potentially would have no longer seen Korea as one of the front lines with the so-called "Communist block". Having the nuclear capability allowed Nth Korea to keep a degree of focus on them -
"Look at me, I have nukes...or might have...I'm relevant!!!"
Funnily enough I have often thought their economic backwardness also provides a deference of sorts. That is, let's say they didn't have nuclear weapons and their conventional capability was deemed able to be overcome by the South (not really unrealistic - just look at how quickly/easily the West was able to beat Iraq, which on paper had an arguably better military than Nth Korea at the time), would the South really want to invade? I don't think so only because of the fact that even if they won they would then have inherited a basket case which would be a drain on the Korean economy. Remember the impact Germany had on their economy following re-unification. This would be much, much more painful. Add to this the benefit the south gets by having the "crazy cousin" to the north. Having this threat, arced up every once in a while with threats and bluster (and with the hint that maybe China would get involved), forces the likes of the US to keep forces there and to continue to supply the latest technologies to them...just in case. Thus IMHO, the Sth Koreans have no real desire to change the status quo.