There isn't one shred of data that these devices actually work, or that TSA's security practices have stopped anything. The TSA does not track anything, so there's no way for anyone to know what the hell the truth is. I suspect that the TSA has not stopped anything since 2001, it's been other agencies (FBI, CIA, etc) who have prevented attacks.
There's also the fact that driving is many times more dangerous than flying, yet flying gets the most "security" (not that I want a TSA pat-down before getting into my car, of course, but it just shows how useless they are).
By the way, the backscatter devices would NOT have detected Mr Underpants Bomber. Oh, and every policy the TSA has put in place has been after someone got through security (e.g. shoe bomber => take your shoes off). Security theater at it's finest. Now, who are the politicians who've gotten donations from Rapiscan et.al. and how do we make sure they're permanently removed from office?