Before we all dive in, I agree that current security is simply designed to pander to the Government cretins and to "be seen to be doing something".
That said, in addition to somebody using stolen of false credentials to get airside, the use of proxy bombs or weapons being carried by individuals under duress (eg by threats to family members) is a well established and preferred terrorist technique for violating secure cordons. However unlikely, it is a threat and 100% screening is the only effective counter measure.