You have probably seen this on TV, a motorcycle racer goes down at more than 100 miles per hour and the camera follows him as he tumbles and slides over hundreds of feet of asphalt, grass and gravel-only to get up, run over to his bike and try to start it again.
Now try the same thing in a pair of jeans and a T-shirt. Cotton pants and shirt shred in a few tenths of a second on asphalt, leaving the rider's flesh to act as brake pads.
Joints as shoulders, knees, elbows and ankles can strike the pavement hard enough to fracture.
The extensive road rush that comes from sliding in pavement is a very serious injury equivalent at a third-degree burn. That's reason enough to wear protective gear when riding. What is the best protection, well... the full leather suit with armor, gloves, boots and full face helmet that the racers wear.
A study done in Munich in 1986 found that motorcyclists wearing protective gear (heavy jacket, gloves, boots etc.) could expect a reduction in injuries of 30% or more.
Most recently a variety of garments and boots have been designed with high impact and protection in mind. Gear made of cordura and kevlar are very comfortable and still delivering good protection. This garments usually have extra armor on the joints and other vulnerable sections. They do not offer the outstanding protection of a full leather racing suit but they are a lot better that a pair of jeans and some light jacket.