The car isn't moving unless everyone is buckled in.
A few years back my wife was in an 'accident.' She ran her '74 (?) Gremlin into a light pole on the side of the road becase she and her best friend were screwing around with the radio.
It wasn't fast, only 15 to 20mph from what she told me, but neither one of them were wearing seat belts. My wife is very petite, about 115lbs now, 4 months pregnant. She flew up and smashed her head into the windshield. Scarred up her forehead a bit, but it mostly just scared her because of the blood. Her friend flew forward and hit her throat on the dash. She was unable to speak for a few days and her spine was somewhat bruised up from the incident.
I was in an accident a few years back with a friend of mine. He was driving, I was in the passenger seat. We were pulling out of a parking space and I hadn't even thought to put on my seat belt yet. We got hit by some (speeding) 15 year old (honestly, he didn't even have his drivers licence or proof of insurance on him) on the pasenger side. My friend was ok, just a little shaken up. I got bounced up a bit, slammed my head into the cieling and door I think. My neck burned like hell after I got out of the car, but by the next day I couldn't turn my head. I had several impacted vertebrae in my neck and upper spine.
From those incidents, I refuse to let anyone ride with me that wont wear a seat belt.