Argument: Scanners do not detect low-density items very well
A British defense-research firm reportedly found that full-body scanners can be unreliable in detecting "low-density" materials like plastics, chemicals, and liquids, which is what the 2009 Christmas "underwear bomber" had stuffed in his briefs.
Steve Chapman. "The Naked Truth about Airport Scanners." Reason. January 7, 2010: "A Conservative member of the British parliament who previously worked for a company making scanners said that "in all the testing that we undertook, it was unlikely that it would have picked up the current explosive devices being used by al-Qaida."