Protection is the number one reason why motorcycle riders wear leather.
Motorcycle riders rely on leather to protect them not only from the cold and wind but to protect them from flying debris and to save their own skin if they should crash or fall off their bike.
If a motorcycle rider crashes, they normally are thrown down the road, sliding all the way and perhaps at a high rate of speed. Without any leather to protect you, your skin will be pulled off all the way down the highway. This will result in an extremely painful injury, not to mention that you will be scarred for life. Why not let your leathers take that punishment instead of your own skin?
Remember, a motorcycle rider does not have the same protection that someone driving in an automobile has. A person in a car has the car itself to protect them in a crash. A motorcycle rider has nothing but what he or she is wearing.
Leather is a tough and resilient material. If you were to crash and slide down the road, if you are not wearing a tough material like leather, then you are far more likely to get road rash which is the pavement taking the skin off of your body. This is why even in hot weather, you will see motorcycle riders wearing leather chaps, boots, leather vests, leather gloves, leather pants and motorcycle helmets as well.
And hey, you have to admit wearing leather is sexy. :)