Airport Scanners - only useful during "busy times"?

USA Today is reporting that a new study is being conducted on the "backscatter" scanners.  (there are 2 types of scanners at airports - this is the one that emits small amts of ionizing radiation.  The other is a millimeter wave scanner and doesn't involve radiation)

Here's a confidence builder - apparently some maintenance records indicated radiation was 10 times higher than expected on several machines (19 out of 40 tested).  Rapiscan, the company that makes the backscatter machines, says their employees who tested the machines were "confused" about the forms they had to fill out, so there weren't really any radiation issues - it was simply "math mistakes".  (note:  there are 247 of these scanners at 38 different airports)

I remain perplexed about airport scanners.  At a cost of $170,000 each, and claims they detect more than the traditional metal detectors, one would think they would be used after they are installed.  Yet this week, I once again encountered a scanner sitting idle at an airport (Houston).  It wasn't a busy time at security - no long lines.  One scanner was in use, the second was not - instead, they were having people go through a metal detector that was sitting next to the scanner.  Ironically, there was a large sign as you entered the security area stating they were using "state of the art" scanning devices.  I forget the exact statement but something to do with safer air travel.


This is a view of the security area after you have gone through the process.  The larger, cylindrical scanner on the right is apparently one of the millimeter wave scanners (grayish white).  This one wasn't in service.  Instead, they were having people go through a metal detector which you can see behind and  just to the left of the scanner. 

In case you're wondering why this issue bugs me, I have a piece of titanium in my leg and if I go through a scanner I avoid the waiting around for a female TSA agent to pat me down and the debate that invariably ensues when they try to stack the bin with my coat, shoes and zip lock bag of liquids on top of the bin that contains my laptop!  When I have plenty of time to get through security, I'll sometimes avoid the backscatter scanners (I admit I don't trust the government's radiation exposure studies). 

A couple of scanner facts I've read in other postings:
-the backscatter, radiation emitting scanners are blue and have 2 walls
-the non-radiation scanners are greyish white (see photo above)
-reports I've read indicate the cost for operating scanners in the U.S. over a 10 year period is $6,000,000,000

If scanners are better at detecting potential explosive devices, guns etc, why aren't they being used?