Note: This is the text of a reply I made to a post at Zaphod's Heads. I thought I would post it here as well. Read the original first to put this in context. This one has all the spelling errors corrected, Doh!
I doubt there are many Western politicians who are big fans of these weapons. The reality is that they exist and the knowledge of their creation cannot be unlearned. It would be a wonderful world if everyone could be trusted to destroy these weapons and prevent anyone from every building them again, but it is naive to think this could ever happen.
For nations, the development of these weapons forces a re-evaluation of the importance of conflict with other nations. Even for the US facing a nation with the weapons is not high on the to-do list. Hence the reason countries like Iran and North Korean want them. It immediately makes them MAJOR players on the world political scene and they believe it moves them to a new relationship with the US. In a way they are right, the US, GB or anyone else would not take military conflict with a Nuke state lightly but it changes the strategic attitude of how to deal with the countries.
A North Korea with the bomb can NEVER attack South Korea. Why? The US strategic policy towards North Korea would have changed to a policy of all options on the table. Not even North Korea is willing to risk total destruction in a nuclear war. North Korea without the bomb has a much freer hand is its aspirations vis-a-vi South Korea because the US is forced into a limited conventional war in which the possibility of high death tolls AND protracted combat could turn public opinion against the defense of South Korea (see the Vietnam War for more info).
In much the same way, the relationship between Pakistan and India is actually much safer today than it was twenty years ago. Before both sides had the bomb, relatively minor differences, as the two countries have recently experienced, would have lead to open, conventional war. Instead the two sides have been forced into communication over their differences.
This is not to say that everything is rosy once a country has the bomb. In the case of Pakistan, if a radical government were to take power the danger to India, and in fact everyone in the world, would be very high. While North Korea could not take overt military action the danger comes from covert action. Supplying these weapon to terrorist groups that share a common enemy (the US) is where the real danger lies. The attempts by the US and others to limit who has access to these weapons is based on concerns over the covert application of them. The Iranian government would quickly discover that using these weapons openly would be impossible but their history of supplying tools to terrorists becomes a huge concern.
Nuclear weapons are not inherently evil (no inanimate object is). What causes so much fear (rightly so) is that if they are used for evil purposes they can far out-reach any other human invention for death and destruction. As long as these weapons are controlled by rational governments the danger they present is minimal. If they fall into the hands of irrational governments or terrorists everyone is in grave danger.