I was a member of the NH Commission on Passenger Restraints. We heard from experts in every field, and of course from asswipes from the FedGov who wanted to give us money.
Anyway, seat belts are extremely dangerous if they are against your neck -- in the event of a crash, the first thing that happens is your carotid artery gets severed, at which point at least you'll die quickly and painlessly and not have to deal with the trauma of a shattered leg.
Also it is uncommon -- but not freakishly so -- for a person without a seatbelt to be thrown clear of a car, who with a seatbelt would have been crushed & killed. This is particularly true of driver-side collisions, generally at intersections. Two different members of the NH House of Representatives gave floor speeches last year, saying that they were alive today only because they were thrown clear of their cars.
Seat belts greatly increase your chances of survival, especially in a modern car with ABS braking and proper crumple zones. All the aspects of safety in the car are designed to protect the human who is expected to be exactly where the seat belt keeps them. But used improperly, fitted improperly, or in just the wrong accident, the seat belt ensures your death.