If he had had the dirty bomb on 9/11/2001, he would have used it...now, he's hiding in caves with more and more of his aides and leutenants getting the "72 virgin" service every day! In Iraq, Zarquawi is getting more and more desperate because the same scenario is happpening and he's more in the open than Bin Laden. Personally, I think we ought to just MOAB the entire border region between Pakistan and Afghanistan because there are more enemies there than everywhere else put together!
Why do you say that? Did the U.S. and Britain use every weapon in their inventory when invading Iraq?
Besides, a dirty bomb would result in few if any more immediate casualties than any conventional bomb of the same size.
The main effect would be to make the area off-limits for a period of time.