The reason I was given - and take it for what it's worth, but it made sense to me at the time - is that in the event of a collision you don't want shards of plastic at or above eye level.
Distortion aside: some earlier extruded plastic windshields are indeed horrible, most will deflect the air about 4 inches over your head, so looking through the plastic is moot. Even in the worst weather (when we all like to go out and ride) I'll crouch down and pop up if the rain gets too bad on my face but I rarely get stuff in my eyes at my height (I'm only 6'1").
Sorry I'm no Physicist. I thought if the bike stopped suddenly, the body on it would continue its forward motion, for a while at least, and anything in its way would try to become one with that body. Ergo windshield in front of face equals face full of windshield.
Now I am sure it is possible to provoke a standing body to rise; i.e. arc over the windshield, however, I would have thought the distance between the face (what? 6-9 inches?) would preclude that from being a factor in a head-on, or a catastrophic front end failure.
Here in France, where we have over a half a million scooters on the road in the city every day, I regularly see riders with broken windshields: particularly those which rise well above the head.
Anyway, I ride mostly naked bikes: my Triumph Trophy 1200 does have a faring and windshield and was delivered with an extra high, eye level unit. It was un-drive-able in cross winds and I bashed my head upon heavy braking a number of times, so I bought the standard cut down model and it works fine. But then that's just me.
I'm no physicist either, but my experience as an emt years back leads me to my beliefs. #1 the windshield will break but it isn't glass, so it shouldn't act as the old type auto shields before plastic and I have never seen a bike shield that didn't break in large pieces. #2. I have never seen a bike shield shatter from face or head impact, it will break from body momentum, but doesn't shatter. I'm speaking on my experience, that doesn't mean freak things can't happen, but I've never seen this to be the case.
Well, like I said, it's just my opinion. I know if you score and break a Plexiglass clipboard you have two handy knives, and I've been cut by the edge of a broken piece of Lexan or other Polycarbonate. I can't imagine a windshield resisting the impact of a face, followed closely by a body, but even if it does and doesn't shatter, it can't do any good. I'm not an expert, I've just followed what I was told a long time ago.
Maybe I'm wrong, I didn't intend to start a polemic on the subject.
I'll still continue to look over mine.
Quack, obviously, as shown by the responses, it is purely and simply a personal choice. As Budoka pointed out out, distortion can be a problem with some shields be cause of the curve of the shield or if the shield is scratched and worn.
But if there weren't so many riders who preferred looking thru, myself included, there wouldn't be so many manufactures selling them.
I'm surprised some moron in congress hasn't tried to legislate it. Remember a few years ago when a bill was introduced to require seatbelts on motorcycles?
On my Shadow I looked through it, on the wing I have the Windbender with the lip. So I have to have it low or high enough not to see through the lip. Most of the time I lower it and look over. I always have some type of eye protection on either way.