After reading this I decided to 'experiment' myself. I took off the left one, with the right one remaining and went for a ride. It was 95 and really humid, I could not tell any difference at first; but, I went home and changed out of my jeans to against what would be considered proper riding gear, a pair of shorts I was immediately informed that the little deflectors do direct a large amount of air AWAY from my knees. The left leg of my shorts was fully inflated, my right leg was as calm as could be. I could not tell the difference in jeans, but the shorts really told the story. To verify I returned the same way I left home to determine if it was by chance the wind form the left side that was causing my left leg to 'inflate', but I had the same experience in both directions.
To make sure ( as any real experiment should be verified) I replaced the left one and the short ride told me they do move air from the top of your knees.