This blog post is a great article on why you should wear seat belts. Although the general idea is well known, this post goes into so much detail of what happens when you don’t wear a seat belt, that hopefully it will scare some more people into complying.
In a collision, you have three or four sub-collisions all taking place in sequence. First, the vehicle hits some object. The vehicle abruptly slows, but unrestrained objects inside it continue at the same speed, in the same direction. Then the unrestrained body hits the interior of the vehicle, and starts to slow. That’s the second collision. That body’s internal organs are still moving at speed until they hit the inside of the chest (or get cheese-sliced by their supporting ligaments—and that’s where you get things like bisected livers or aortas). The fourth collision is when the bowling ball you left on the rear deck hits you in the back of the head, because that continued at the same speed in the same direction. Newtonian physics: Learn it, live it, love it.
You definitely need to read the whole post. It goes on and on in this vein. If nothing else, it is very medically informative.
Over the years, on multiple occassions my friends or acquaintances have been involved in accidents were some of the occupants of the car were not wearing seatbelts and had very serious injuries (and in some cases, died). But people in the same accident who were wearing seatbelts walked off with minor injuries. I have myself driven my car head-on into an immovable object at 120kmph on the Pune-Bombay highway. My car suffered to the tune of 1 lakh rupees. I walked off without any injuries.
As the article points out, in many places, wearing a seatbelt is now required by law. Non-compliance is punishable by a small fine in some cases, and in other cases, death.
(I found the article from this Boing-boing post.)