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.