The risk is
infinitesimally small because:
1) There just aren't that many terrorists. It's like serial killers and child sex slaves: the number isn't zero, it's just very, very very close to zero. As Bruce Schneiner says: If you read about in the paper, it's not worth worrying much about.
Airplanes rarely fall out of the sky (in the US or anywhere else) because very few competent people actually want to blow up airplanes, not because airport security is particularly useful.
2) Most of the very small number of (wannabe) terrorists are "not the sharpest scimitars in the shed" to use
Gene Healy's turn of phrase. Stupid terrorists are easy to catch and not much of a threat even if you don't.
3) So we're left to worry about a tiny number of competent, well-financed terrorists. Having 60,000 high school dropouts groping 700M air passengers a year is unlikely to uncover any of them. Even if it did no harm otherwise (and it does), the resources would be better spent elsewhere.