That argument sounds nice until you consider the potential for accidents in the automated systems that run it all, the potential for human error in already tense situations, and consider the destabilising knock-on effect they have in a region. Although maybe you think these are good things or at least will make some blasé smartarse dismisal of them all.
The MAD argument also fails when you consider that the Americans are quite adamant that they would use their stockpile in either a large scale First Strike or tactical devices in a minor confrontation (there is also the trivial matter of who exactly would be threatening the USA). Again, plenty of opportunity for arsehole-ishness here.
Just my $0.02, but I think if terrorists ever get their hands on a nuclear device it'd probably be something like what David Hahn built in his mums backyard in the 1990s:
http://en.wikipedia.org/wiki/David_Hahn