This is why it's stupid

Spy Agency Data After Sept. 11 Led F.B.I. to Dead Ends - New York Times

"More than a dozen current and former law enforcement and counterterrorism officials, including some in the small circle who knew of the secret program and how it played out at the F.B.I., said the torrent of tips led them to few potential terrorists inside the country they did not know of from other sources and diverted agents from counterterrorism work they viewed as more productive."

There is opportunity cost associated with the illegal warrant-less domestic spying. Time and effort spent following pointless leads is wasted (and could have been spent elsewhere).