For many years my part-time jobs were serving as a Fire-fighter, EMT, and Emergency Rescue Technician on two FD's in the Chicago area, one of which we covered a few miles of I-55 into Chicago. We handled far more vehicular accidents than all fires combined as I recall, and I just can't see how or why anyone would argue against the survivability factor being more in your favor in most crashes if you are wearing your belt. If an adult feels that their personal freedom trumps common sense regarding seat belt use - well, that's one thing. I'll say my piece one time to them, but after that I don't argue with 'em. No use.
It really burns my butt to see somebody with a two year old on their lap, or a toddler standing on a seat in a moving vehicle. It's awful to have to search through a wreck, or in the area of a wreck to find a little one that had no chance.