Many of the old school players... Here's one:
I've also seen Bustamante and Reyes do it on occasion. Sometimes they do it as part of coming down on the shot, as
@straightline mentions above, some do it as a sweep.
It's not necessarily a hugely exaggerated windmill for everyone, but air strokes are something I notice a lot of players doing, especially if they've been sitting. My thoughts are it helps them get their stroke calibrated and find the balance point for a particular shot. Air strokes seem to help with the visualization process. The more out of the ordinary or important the shot the more it seems to happen. It's a great way to shut your head up and visualize.