The main purpose of a polygraph is to help the authorities look for suspicious answers and then grill the individual on those points. If they are lying and belive the machine indicates that they are lying, they often confess.
A polygraph is basically a confession tool and nothing else. I would never consent to take such a test because if you pass it, it does NOT exonerate you and if the result looks like you are nervous (and who wouldn't be in that situation) then it brings on the appearance of guilt. There is no benefit in taking a polygraph test.
Gum
Same for taking the 5th Amendment. Why have rights if we are not going to exercise them?