Steven Maasland opined, "They aren't much use if any under 20 mph...", but later he said that the fairings "protect your privates and hands in very cold weather". That is precisely when I would use my fairings (I have 2). Sort of like putting the windscreen on the tractor to plow the lane of snow, I guess. Reduces self-inflicted windchill factors on the bikes.
Ken Wehrenberg, Hermann, MO