Nuclear weapons, by their very nature are NOT defensive.
Regardless of whether mutual assured destruction is a sound military doctrine or not, the fact of the matter is that nuclear weapons have resulted in the deterrence of conventional aggression between states belonging to the developed world.
Throughout history, the great powers of Europe have regularly gone to war with each other; that is, up until 1945. Since then, there have been no major conflicts at all between the major states. These are the same states that suffered unbelievable devastation and losses due to World War I, and still that wasn't enough to prevent a second World War from occurring a short time after.
To say that the nature of government underwent a fundamental change in the year 1945 would be a ridiculous claim. A more reasonable claim is that, for the first time in history, the costs of committing acts of conventional aggression (between major powers) have become so great and terrible as to dissuade their execution.
Individuals supporting nuclear abolition have good intentions. After all, nuclear weapons are horrible weapons of destruction easily capable of causing our complete eradication. While that's true, I assume that, based on our own long history, that complete nuclear abolition would usher in the return of conventional aggression between major powers, leading to major conflicts or even a new world war.
Here's a very short paper on the subject from a foreign-policy think tank in DC: http://www.carnegieendowment.org/files/Miller1.pdf.