Dirty Bombs Unharmful?

It would appear the our government is overreacting to so called "Dirty" Bombs. At least according to the Tech Review at MIT, dirty bombs, which are made up of a normal explosion spreading pre-existing radioactive material, would harm people far less than just a really big explosion. There is no fission or fusion causing a chain reaction in these explosives. Read it for yourself here, and they have a very convincing argument:

http://www.technologyreview.com/articles/wo_muller062304.asp?p=2