I like the way this guy thinks about security as a trade-off:
In the week United States-led forces invaded Iraq, the
service was receiving a hoax bomb call every two or three hours, but
not one aircraft was delayed. Security experts decided the cost of
halting flights far outweighed the actual risk to those on board.
It's a short article, and in it Mark Everitt, General Manager of the
New Zealand Aviation Security Service, says that small knives should be
allowed on flights, and that sky marshals should not.
Before 9/11, New Zealand domestic flights had no security at all,
because there simply wasn't anywhere to hijack a flight to. [Schneier on Security]