There is one valid reason why x-ray was chosen. They clearly produce superior images - possibly an order of magnitude better than MMW. This makes detection of stuff easier for their high school radiologists. They clearly have better "detection" capability.
Actual image here: http://epic.org/privacy/body_scanners/Body_Scan_Pic.jpg

The MMW's really can't see crap, but because of reflective properties, stuff shows up "bigger than life" so things aren't too hard to spot, despite the crappy resolution.

The x-ray has a "chance" of detecting powder if the high school radiologist is really on his game. The MMW has no chance of seeing powder, unless is is packed dense or in a container or bag. Bound in underwear by spray adhesive or such, it wouldn't have a chance of detecting it, in my opinion.

I think they recognized the problem with x-rays early on, but these were the only 2 companies that met the procurement spec and the Rapiscan is clearly superior.

The ATD issue hopefully will kill off the Rapiscan, as their auto-detection software is clearly not ready. The MMW ATD has been in use for 2 years at AMS, and the latest revision passed TSA's tests. Hopefully pressure will cause them not to buy any more Rapiscans, but it'll take a second round of politics to get the existing ones scrapped. We can only hope.

BTW: Successful SDOO at DEN today. Kept belt on and wallet in back pocket, no problem. Young female BDO asking destination questions. Asked where is was going, said "somewhere warm" and ducked through WTMD before any further interaction.