They act as a deterrent, to keep other nations from using their own weapons out of fear of our own retaliation. I entirely agree with you that it would be wonderful if ALL of these weapons could be destroyed, but the fact is is that the first nation to do so puts itself at such great risk and military inequality to its possible enemies as to prevent anyone from ever making any such unilateral move, and arms treaties and mutual reductions do not appear to be an option with some of these countries (namely North Korea).
And although we may be more advanced militarily and in other technical aspects, I believe that it is extremely insulting to the Iraqi people to claim that they are not as "civilised" as we are.
On another note, our use of atomic weapons to end WWII was not, in my mind, an irresponsible use of such power. Any invasion of the Japanese mainland, based off of our fighting in Okinawa, and reinforced by reports we got in Japan itself after the war, would have created enormous ammounts of allied casualties, as well as virtually wiping out the civilian population of Japan (which had been instructed by the Emperor to fight to the death). It was terrible, but neccesary.