Originally Posted by
Trollkiller
In a word, reliability.
With the walk through MMW you have a controlled environment that is tuned to allow the sensor to see through clothing.
With the passive MMW it relies on ambient radiation. Because the ambient radiation varies by location the reliability of the machine is lower.
Sorry, but I don't accept exchanging reliability for indecent exposure. Also, both systems are unreliable, because they cannot detect anything within body cavities. These things are a total fail.