The blackmail and sabotage argument, as near as I can tell, is a farce because our enemies and "friends" are engaged in massive corporate espionage. If it were a real concern to them than they would use thier powers to require the same level of scrutiny on the private sector.
In addition to that they say be truthful so you don't have anything to hide to prevent possible blackmail by foreign powers, but in reality it's the truthful ones who don't make it through the process.
I've come to the conclusion that it's really just about having an excuse deny employment to "people not like us."