Earlier quoted context omitted.
Dirty bombs aren't an issue. You might want to ask yourself why we've never seen one used, especially if you understand how easy it is to obtain the necessary materials. IIRC we've only caught a handful of devices (none exploded). Also we have to consider when it explodes, it just doesn't make enough radiation. It is pretty difficult to generate enough radiation that would actually cause an increase in cancer rates (…
Terrorists are not known to be the most logical people, to be honest. They are going to try anything that can generate terror in large scale and a dirty bomb is a very, very terrorizing thing to general population. General population thinks anything with radiation is panic worthy, even though there isn't that big of a deal.
godelski is arguing that terrorists who can be convinced to try and use dirty bombs will be less effective than if they had chosen other, more damaging, options.