Contrary to popular belief a properly designed windshield will reduce wind resistance, not increase it. The human body, as it sits on a scooter is not very aerodynamic. A properly designed and sized windshield will help overcome this problem. Hence the reason many riders report an improvement in speed and gas mileage after installing one.
I will agree, on the other hand that a windshield may cause handling issues under certain conditions. In cases of severe crosswind the windshield could behave much like a sail.