Why Do We Care About Dirty Bombs?

I mean I know they've been a part of the scare-tactic arsenal for some time now (the terrorists have dirty bombs! like omigod!!), but hasn't the whole dirty bomb concept been pretty well debunked by now? Apparently not. It seems our border security is being tested based on whether or not they can prevent a dirty bomb from entering the U.S. Honestly, do these people not know how to use wikipedia?