If you can't pass a poly, regardless of how innocent you are, that's a personality indicator that puts you at risk for not being able to safeguard critical information.
No. It does not. It simply shows a biological response of any significant emotion at the time.
You are right as to what it indicates. And parent poster was right as to the implications of that indication. If you cannot control your physiological reactions to different stress inputs, you're probably not the type they are looking for. Source of the stress is irrelevant.
If a polygraph were actually effective, why would you ever need to give someone more than one?
You mistake "effective" with "perfected". If polygraph were perfected, there would be no need to repeat. They aren't perfected.
Why is it that the only result the NSA will "accept" is that of a pass?
What is the purpose of having a test if not to check whether applicants can "pass"?
Out in the real world, sometimes you have to sacrifice "ideal" and "perfect" and settle for "this works better than whatever else we have". You put a bunch of "this works" tools together, and hopefully you can draw an image of a candidate that is comprehensive enough to make an informed decision about their suitability for the job. Is a polygraph perfect? No. Does it operate in a vacuum? No.