The purpose of the TSA is not to catch every piece of contraband; it is to reduce "aircraft attacks" [EDIT: by aircraft attacks, I maan "attacks against the airplane itself," that is, bombings capable of bringing down the plane. Hijackings are not really feasible anymore]. For that purpose, it does not have to be 100% effective, or even 95% effective, or even 90% effective; it just has to raise the apparent level of difficulty high enough such that:
a) An attack looks like it will take so much effort that the terrorist is no longer interested (terrorists are lazy, too) or capable. Attack frequency is inversely proportional to effort involved. If all a terrorist had to do to down a plane is click on a link in a web-browser, we would see catastrophes daily; if the sophistication of the attack is such that it takes months of training, the frequency drops by orders and orders of magnitude.
b) It forces a terrorist to get help from other terrorists; while a solo terrorist is hard to detect in advance, terrorist networks and their communications are routinely infiltrated by law enforcement and intelligence.
In this respect, pat-downs and body scanners force terrorists to up their game, which, in turn, discourages attacks from ever taking place. The Israelis also use this method [1] but they apply it more selectively based on the background of the passenger, which saves resources and, while politically difficult, is a much better approach.
[1] "At Ben-Gurion, some passengers have been searched so thoroughly that they have had to walk through the terminals, the gates and up to the doors of their planes with no handbags, wallets or even shoes." http://www.cleveland.com/world/index.ssf/2010/11/tsa_alterna...