"Secure Flight is a passive system. It waits for the bad guys to buy an airplane ticket and try to board. If the bad guys don't fly, it's a waste of money. If the bad guys try to blow up shopping malls instead of airplanes, it's a waste of money.
"If I had some millions of dollars to spend on terrorism security, and I had a watch list of potential terrorists, I would spend that money investigating those people. I would try to determine whether or not they were a terrorism threat before they got to the airport, or even if they had no intention of visiting an airport. I would try to prevent their plot regardless of whether it involved airplanes. I would clear the innocent people, and I would go after the guilty. I wouldn't build a complex computerized infrastructure and wait until one of them happened to wander into an airport. It just doesn't make security sense.
"That's my usual metric when I think about a terrorism security measure: Would it be more effective than taking that money and funding intelligence, investigation, or emergency response -- things that protect us regardless of what the terrorists are planning next. Money spent on security measures that only work against a particular terrorist tactic, forgetting that terrorists are adaptable, is largely wasted."