The conclusion by a friend's father, when he went from Minneapolis to Chicago and back, was that the TSA only pulled a side young females (20s & 30s) for full body scans.
During 2010 and the first half of 2011, my significant other (female) was randomly selected for the scanners 11 out of 12 times. The 1 time she wasn't selected was because the scanner was not in operation. I (male) was randomly selected 0 times.
Every time she asked for a pat down, the female TSA worker (who would be performing the pat down) would scowl at the young male worker. It seemed that female pat downs were in higher demand, making her job harder.
Now that more scanners have been installed at the airports we frequent, the TSA workers try to send almost 100% of the people through the scanners. Though, my significant other now gets directed over to the metal detector if there is a long line for the scanner.
Well, anecdote doesn't tell you how often this happens, but it is an existence proof. I think I would expect a priori that some TSA agents would be trying to do a good job and some would just be in it to letch on passengers. The question is how prevalent the later is compared to the former, and why the TSA isn't trying to distinguish them themselves.
This has been my experience with my family, too (wife and two young kids). We've stressed about the pat-down v. scan each time we've flown, but the scanners haven't even been on when we've flown out of PGH.
My regular comment has been, "I don't mind the scans, but I'd appreciate it if they would stop giggling."