Wouldn't it make more sense to disconnect them from the internet than to try and shutdown communication to/from some sections of the world? The ability to momentarily create a strong perimeter defense doesn't make much sense to me- it would only work against an attacker that didn't plan for it.

As soon as the world knows you have a kill switch, any organized attack will have domestic components, and which means that you still have the possibility of the president screwing up the normal economy, but no additional defense.

Regardless, it would have an effect on communications.

And so, why would it be a good idea? That's still what I haven't hear. (It also sounds like your justifications differ from Leiberman's–he's interested in the communications aspect.)