With the knowledge of how to make them being out there, if everybody agrees to ban possession of them at the state level, all that exists is the potential of a terrorist group getting its hand on or manufacturing one. All they serve as now is an icon of mutually assured destruction, nobody is ever going to use one on another country unless we have already reached an "end of the world" scenario.
Like guns, nuclear weapons are a tragic invention. But, there is no way to completely rid the world of either nuclear weapons or guns. Someone somewhere with enough access to technology and enough money will be able to create a nuclear weapon. That is why stable nations must maintain some to provide a credible counter-strike threat.
Without nuclear weapons, we are presented with the threat of single world power, which is risks safety, and presents danger.
Without nuclear weapons there is reasonable fear of disobedience which results in more fear. With more fear we are presented with threats of danger, as people will do whatever it takes to be safe and protected.
Without nuclear weapons the possibility of longer war roams ever present, therefore causing threats of more danger.
With threats of danger comes a lack of security. Security is something we all appreciate, I'm sure. Therefore without nuclear weapons we lose something we greatly value, security.