See (amongst many) Bruce Schneier on this subject:
http://www.theatlantic.com/doc/200209/mann

To forestall attacks, security systems need to be small-scale, redundant, and compartmentalized. Rather than large, sweeping programs, they should be carefully crafted mosaics, each piece aimed at a specific weakness. The federal government and the airlines are spending millions of dollars, Schneier points out, on systems that screen every passenger to keep knives and weapons out of planes. But what matters most is keeping dangerous passengers out of airline cockpits, which can be accomplished by reinforcing the door. Similarly, it is seldom necessary to gather large amounts of additional information, because in modern societies people leave wide audit trails. The problem is sifting through the already existing mountain of data. Calls for heavy monitoring and record-keeping are thus usually a mistake. (”Broad surveillance is a mark of bad security,” Schneier wrote in a recent Crypto-Gram.)