None of my kids were ready for a full time booster at age six. Only one of them was due to a special need issue.
In much of the US, people spend a crazy amount of time in the car. Hence the fanatic popularity of cupholders in our vehicles. On Wednesdays, I pick DD2 up from school and drop her for riding practice, get DD1 and take her to the craft store for supplies for the class she teaches, then to the class, get DS from school and drop him off at church, go back to get DD2, then back to get DS and DD1 then hit the grocery school on the way home. This routine takes seven hours. SEVEN HOURS. While this is my busiest day, 3-5 hours in the car a day is pretty normal for where I live. When I had 4-6 yos that had to tag along everywhere all day long, it just wasn't reasonable to expect them to be capable of sitting in perfect position for that amount of time.
As far as discipline goes, when I'm driving, I need to be driving. I turn into a very poor driver when I'm trying to get a wriggling child back into position with one hand while navigating a 10 lane 4 freeway interchange.