The official reason the backscatter machines were removed is, IIRC, there was a congressional mandate that the privacy software (gumby screen) had to be implemented by a certain date. The manufacturer was unable to implement the technology so they were scrapped.
There was speculation that potential cancer could be the real reason - especially for the TSOs that had to stand next to them all day.
There is a valid medical benefit to giving up some dignity to have a medical exam. Assuming the position and being felt up by TSA workers hired from pizza box applications is just theater.