Isn't a kill switch sort of like shutting the door after the horse left the barn? So a thief steals a phone and finds he can't use it, then what? The phone is already stolen, he's not going to return it even if he could. Plus, doesn't he now have to steal more phones to make up for it?
It might work if every single phone had one, but there are far too many already that don't. I'd think we'd need a software solution that could be rolled out to existing phones. Maybe an alarm that would go off if your phone was ever separated from your iWatch for an extended period of time.