Usually there is not a lot of fast moving air at the base of the windshield as is apparent when driving in the rain with RainX on the windshield. You can see how little of the rain moves around at the bottom. It is also a high pressure area (caused by low air velocity) which forces air into the heater box and air intake vents.
During some very heavy gusts a few days ago, and while driving home in my Rollerskate (festiva) i noticed how much the wind was yanking the wiper blades around during gusts, and wondered myself what it would benefit. Already have a grill block, lower radiator/condenser intake block (partial) and air dam. getting ~40 with a fubar carb and insane blow-by (Dead rings)