One advantage to having motorcycle windshields is that it blocks the wind and makes for a better and more comfortable ride. Having windshields there to block bugs flying at the face and rocks being thrown from the vehicles in front is definitely an advantage. No one wants to drive down the highway and suddenly taste a bug or see one spat on your cheek just below your eye. Windshields definitely offer the advantage of better obstacle blockage.

A disadvantage would be visibility. While it is great to have something to block things flying at your face, it isn't so great when all that builds up on motorcycle windshields and blocks visibility. You can't drive safely if you can't see where you are going.

Another advantage is the aerodynamics of motorcycle windshields. They allow the air to flow evenly up and over the motorcycle. Having better aerodynamics with windshields can improve gas mileage and with the gas prices sky rocketing, any amount saved in gas is always an advantage.

A disadvantage would be the glare given off by motorcycle windshields. Not all windshields give off a glare, but when they do, they can be blinding. It is hard to drive when you are being blinded by the glare, likewise, it is hard for other drivers to drive with a glare.

Another disadvantage is the weather. In some weather, it is better to see without motorcycle windshields then it is to see with them. They can make it hard to see in foggy or wet weather. Just like with glasses, windshields can fog up, attract moisture and have ice. Any of these conditions can make it difficult for visibility.

If you get the wrong size it could definitely affect visibility. Trying to look over the windshield or under the top of the windshield can be annoying and dangerous. If you don't look entirely through or entirely above your motorcycle windshields, you could have problems with blind spots due to the line in your line of vision.

One consideration that would give the best of both worlds would be removable motorcycle windshields. This would allow you to place it on the motorcycle when going on long trips, and remove it on bad weather days, or when visibility is poor to begin with. It can add to the aerodynamics when you ride in conditions that would affect the mileage and speed, such as uphill and curving roads. Also you may consider having the windshield tinted to reduce glare and increase visibility. Be careful how tinted you have the windshield though, because in dark conditions, it may make it hard to see if they are too dark. Glasses have anti fog coating and you can get the same coating for your motorcycle windshields.