> the only effect it was likely to have was to make things flow a little bit less smoothly for all the other poor folks standing in the long line behind me.

Why should it have this effect? When I opt out, they have me stand in a little area off to the side until a scanner is ready for me. I don't hold up the line, and, as far as I know, most people don't even notice that anything is happening (except that some wonder curiously why I am electing to stand around, apparently for recreation, without belt or shoes).