there are 100 stories either way...
About three months ago I stood on the end of my driveway and watched two kids flip a Porche @ 100mph (convertable) down our road. My neighbor and I were first on the scene and both kids lived (one walked away). The car flipped 5 or 6 times and rested upside down. I know for a fact that they would have splatted on the side of the house had they not work a seatbelt. As I stated, the choice is yours. I won't ride with anyone that doesn't wear the belt.
Here's a few to think about...
http://www.youtube.com/watch?v=N0B6-vi6PyA
This last one says it all....
http://www.youtube.com/watch?v=uuc0aUDAz6w