In fact, that's one of the advantages of thorium reactors and fuel cycles.

http://en.wikipedia.org/wiki/Thorium_fuel_cycle#Advantages_a...

Thorium can't, as far as I know, be used for bombs. Unlike plutonium or uranium.

And there you have the real answer why it is not used and why uranium/plutonium is. It has very little to do with the economics and safety of energy production.

It can still make a nasty mess, though.