I have a friend that contents that seat belt laws actually cause more acidents. They give people a false sense of security. He contends that accident rates would go down if you removed air bags and selt belts and replaced them with spikes on the dash boards. Your friend is a class 1 moron. The real advantage of a seat belt isn't what it keeps you away from (although that's what is sold) but what it keeps you near. The number of minor accidents that could have been much worse if the belt hadn't kept the driver at the wheel is pretty amazing. Texas Department of Public Safety did a study on it a few years back, and it should be online somewhere. Fascinating stuff.
For this reason, I don't equate seat belts with motorcycle helmets, at all. A brain-bucket is just for me, and me alone. If I don't wear one, tough, live with it.
On the other hand, if a seat belt can keep me at the wheel, and in control after something has gone wrong, that could save your life. Also why I support compulsorary eye protection for cyclists.
I didn't say I agreed with him. I road race (on tracks) Porsches for a hobbie. The best upgrade to a car is adding a 5-6 point harness. This keeps you in front of the steering wheel without having to use arm strenght. This allows you to have precise control of the car. This would be great if all road cars had them. However since many people find the simple current seatbelts to complicated to bother with you would never get them to use a multi-point harness system.
What insurance companies need to do is to start limiting their coverage if the driver is not wearing their seatbelt. You can't base rates on not wearing seatbelts because they would never know. They would ask during the quoting process but who is going to say they don't wear seatbelts if they know it is going to cost them more.
The same goes with motor cycles. You can wear a helment if you want. However, if you are in an accident you will not be covered for any injuries caused by your lack of helmet wear unless you told them up front that you will not be wearing a helmet and you accept the higher risk and higher payments. Thinking about it this could aply to seatbelts also except that there is the additional risk of causing the accident because you found yourself on the passenger side of the car with no control because you slid across the front.