If anything, I think a bug deflector would encourage more rocks to hit your windshield. A bug deflector will deflect bugs using the airstream. Unfortunately, rocks are heavier and end up getting directed into your windshield instead of over it.
I do a lot of highway driving and hate the stone chips on the front edge of the hood, but I would rather have the chips there instead of using a bug deflector and have those rocks hit the windshield instead.
