No one today questions the efficiency of x-ray technology to detect and display the hidden objects. Single, dual or even multiple view provides the ability to find everything what is concealed.
However, it is the operator, who makes the final decision and allows the person to take the belongings and go or send them for the manual check.
The operator may be tired or scattered. Even if he follows the regiment and takes regular breaks. He may not identify an object on the x-ray image clearly. He may not identify an object as dangerous if it is a part of some bigger mechanism and take a critical “false negative” decision. In worst case he may even be corrupted.