Wife was driving and crashed into the side of a semi, she had an air bag but I didn't.  I was wearing a seat belt and if I hadn't I'd have been through the windshield.  

A few years ago a kid was killed when they fell out of the car on an embankment  and the car rolled over them.   Simply wearing a seat belt would have resulted in few if any injuries.

Many years ago another kid had the same thing happen. 

My own kid rolled his pickup, but he had his seat belt on and escaped with no injuries . . . from the accident.  I on the other got smacked in the head when the cable was flipped over the truck to pull it back upright.

Yes there are instances where a seat belt wouldn't have saved them, but there are plenty where it would have.  I've had seat belts since 1963.