The recent media uproar over new U.S. airline passenger inspection rules (either a body shape revealing scanner or an invasive physical inspection) has again raised the issue of what kind of airport security is appropriate to the task of keeping terrorists away. The main criticism is that many of the security measures adopted since September 11, 2001 have been more for show than for effectiveness. An increasing number of potential passengers are no longer flying because of these new methods, which have, so far, not caught a single terrorist. This is described as a sign of how effective the new measures are. But the new techniques would not have detected the "underwear bomber" of last Christmas, who secreted explosives in his underwear. Moreover, there have been many cases where passengers got weapons past security, usually by accident.
The current methods also do nothing to deal with other dangers. Chief among these is checked baggage, which does not get examined to the degree that carry-on baggage does. Then there is the opportunity for a suicide bomber to set off their explosives in a line of passengers waiting to be screened. This would shut down American air travel as effectively as a bomb going off in an aircraft. There is also the problem of weak security with airport workers, who are only checked randomly. Again, there have been many accidental penetrations of security in non-passenger areas, and lapses in screening of applicants for airport jobs.
Finally, as was demonstrated recently with two airfreight bombs coming out of Yemen, air freight in general is very vulnerable, especially since a lot of this freight ends up in the cargo holds of passenger aircraft. Thus for determined and well organized terrorists, there are more vulnerable areas to get a bomb onto an aircraft than via a passenger. These more vulnerable areas are given less attention partly because they are less visible, and thus provide less visibility for politicians seeking to demonstrate that they are "doing something" about airline security.
Given the large number of airlines all over the world, and the failure of Islamic terrorists to get bombs on any of these aircraft, even though many different types, and degrees, of passenger screening are used, indicates that there are less antagonizing, and more effective types of screening available. Some of these alternate methods, of course, are not practical for the vast American passenger airline system. The Israeli system, which depends on lots of intelligence work, and highly trained screeners who profile and interview passengers, is too expensive for American use. But the Israeli system is more effective, particularly in light of the fact that Israel is the most desirable target for Islamic terrorists.
Another issue is the small number of terrorist groups actually capable of planning and carrying out an attempt to bomb an airliner. So far, attempts are being caught at the planning stage, which is largely a matter of intelligence and police work. But politicians get little praise for intel efforts, while airport security is very visible. The biggest problem is that airline security is more of a political than security issue. The U.S. is willing to cut intelligence agencies in order to provide more "security theater" for passenger screening. For a politician, it's better for their careers, even if it puts the passengers at more risk.