Doesn't the polygraph machine just play a role similar to "the manager" in a used car negotiation? Just like the salesman can leave the room, get a coffee, never actually talk to anyone, and come back in and say "Sorry the manager says I can't go that low" and lots of people will buy it - the polygraph interviewer is "saying" to the interviewee (in not so many words) "sorry pal, I'd love to believe you, but the machine says you're lying - my hands are tied"

Seems like it's a useful prop for manipulating people, and in that role it really is effective.