Agree with Ultralights. The probability of any terrorist attack is extremely small. The consequences of an attack can vary but are usually tiny compared to non-terrorsist threats to humanity.
All the same, terror is involved, so we need to do something! Gotta be able to sleep at night.
In the case of air travel and use of aircraft as weapons the government has a responsibility to do everything reasonable to prevent an associated terrorist attack. IE, pax screening. An organised terrorist group will always find ways around these security mechanisms but they are required nonetheless. You'd assume that domestic intelligence agencies post Sep 11 have a new focus on pilots from trainee to airline captain. The spooks are the primary line of defence against future terrorist attacks.
Cynics may (self included) doubt the validity of recent arrests of terrorst suspects in Australia and the UK, but this is evidence of an active intelligence service. All we can do is hope their net is cast wide.