It seems like they would end up catching alot of people looking suspicious for non terrorist reasons- like lying to a spouse about a 'business trip' as they fly off to meet a lover or people in slightly altered states of consciousness. My last international flight, I was coming off of mushrooms, hadn't slept in a couple of days, and was obsessively looking for a bathroom to rinse the coating off of pain killers that were legal in a country I had just been in, though not the one I was flying out of. When my flight got canceled, and I missed my connecting flight to Tokyo, I'm sure my face showed panic, fear and paranoia, but I certainly wasn't a danger to anyone on the flight.