The problem with them is fundamental: "You will only get someone desperate enough to take your exam."
If the person is qualified they will be swimming in opportunities and will likely throw your exam directly in the trash heap. If they aren't you probably don't want them working for you. I suppose these might work if the entire industry colluded on it but then ... prisoner's dilemma.
Maybe a more useful indicator is weeding out the candidates that didn't say "no thanks." Cause really why are they so desperate for a job and why do they have all this free time? but then ... ethics.