No other safety innovation in the history of the automobile has saved more lives than seat belts. In addition to that, they are cheap and very unintrusive. Without your seat belt on, the other safety mechanisms in your car do not function properly - for example, an air bag does not help you if you are not in the proper position.
Vehicle accidents cause billions of dollars of cost to society. If you expect society to pick up the pieces of your life in the event of a catastrophe (and everyone does), then the least you can do is cooperate in such a minor way.