For instance, you couldn't use such a weapon against China or Russia without risking an immediate nuclear retaliation. I think that would also be an issue if you were to use a hypersonic possible trans-atmospheric weapon, like the Air Force wants to produce. You are really going to have a limit on who you can use it against for fear of a nuclear response.
That being said it might be nice to have such a weapon if you found out that Osama bin Laden was going to be at location x for the next half hour. Deliverable anywhere in the world in 20 minutes. It would be faster than even the most psychotic Domino's driver.
Hmm are there means of telling whether a given ICBM is carrying something radioactive while it’s still way out there?