If I ride a naked bike stock, it generally gets worse mileage, than it does with a windshield installed.
My latest bike I noticed this on was an old Honda Nighthawk I have, stock is not as good as when I put on a large aftermarket windshield I bought for it.
I took the windshield off for a ride, and MPG dropped again, put it back on and up went my mileage.
Not saying all bikes are the same, but in this case it adds comfort, i eat less bugs, and get better MPG with it on the bike.