and it has nothing to do with the law. My rule is you must fit properly in a seatbelt or you are in a booster. My 9yr old is still in a Husky. The law here is 8 I believe, but many kids aren't in them after 6. I just tell the kid or the parent that they must sit in a booster in my car (I have an extra). It has never been a problem. (I use the excuse that we have been hit 3x with kids in the car.. which we have.)
Now if there was an emergency of course I would if they were boarderline. I would put the biggest kid out of it (even if it were mine) if it were a short trip. We had car trouble once and my DD was 4 at the time. We rode home (in a strangers car..) without one. I felt terrible but I called everyone I knew and couldn't find a ride.
Great story.. a teenager that lived near where I broke down checked my car and discovered what was wrong with it. (It was smoking, and apparently a belt had snapped.) He said I'd never get it home. I made a bunch of phone calls but couldn't reach anyone. He installed my DS's roundabout in his car, but there was no way we were taking out the Husky. He drove us all home.