Intelligence agencies are understandably suspicious of so-called "walk-in" sources: those who haven't been "pitched" by one of the agency's own intelligence officers, but who come in off-the-street volunteering their services. There's a natural suspicion that such a person is more likely to be a plant, a double-agent, sent by a foreign intelligence service or an organization such as Al-Qaeda. The CIA has a long-standing practice of polygraphing walk-ins, and it appears that it has in the past
relied upon polygraph results to discredit them.
However, looking at the historical record, we see that two of the Soviet Union's/Russia's most valuable American sources were, in fact walk-ins: CIA agent Aldrich Ames and FBI agent Robert Hanssen (who actually might better be described as a write-in and phone-in source to the Soviets, as he never physically met his handlers, and his true name remained unknown to them). These two walk-ins disclosed to the Soviets/Russians details of virtually every U.S. intelligence operation being run against them.
But what if the Soviet and Russian intelligence bureaucracy had mulishly insisted that before crediting any intelligence provided by these unsolicited, volunteer spies, they would have to pass polygraph tests? Might Ames have shown consistent, specific, and significant reactions when asked by untrusting KGB agents whether anyone from the U.S. Government had instructed him to approach them, just because he was nervous about the possible consequences of not being believed? Of course! And had the Soviets/Russians believed in lie detectors, and had Ames failed to pass their "test," they might well have lost one of the most valuable assets they ever had.
Similarly, Hanssen, who refused to meet with his controllers, or even divulge his real name to them, might have been rejected from the get-go for his refusal to submit to polygraph screening.
It's worth noting that a double-agent sent to a U.S. intelligence agency as a walk-in is presumably more likely to have been trained in polygraph countermeasures, and to understand that the "test" is a pseudoscientific sham, than a bona fide walk-in. Czech double-agent
Karl Koecher, who "defected" to the United States and eventually infiltrated the CIA received such training. It may be the case that the double-agent is more likely to pass the polygraph than the legitimate walk-in source.
It will probably never be known how many golden opportunities the U.S. intelligence community has lost because of its misplaced reliance on polygraph results. But by terminating this foolhardy practice now, it can prevent such lost opportunities in the future.