If they also took a picture of everyone standing sideways wouldn't that solve this problem?
Seems like it would, but that would more than double the amount of time it takes to pass through the scanner already . And the scanner already takes MUCH more time than walking through the X-ray -- not only does the scanner process itself take 10 times longer, but you also have to empty pockets more thoroughly (i.e. no wallet or anything in pocket).
So it would basically wreak havoc on the throughput of all those security lines...