Your example of how your 12 year old daughter was taller than your wife is clearly a case where the guideline no longer applied (assuming your wife is not so diminutive to be in the size class of a small child, which I am certain you would have acknowledged if it were the case).
The number one reason why we need fewer regulations is that our esteemed regulators only know how to make edicts of an exacting nature. They do not know how to define the requirements based on a set of guidelines with the flexibility to account for the exceptions to the general rules of thumb. Hence we have young children who are big enough not to be endangered to the degree a small child would be when riding in the front seat of the car, but it still violates the regulation. Fortunately, many jurisdictions deal with absurdities like that by granting the law enforcement officers the latitude to use their best judgment in determining whether or not to check the age of the child. That is an imperfect solution, but it is a workable one and one that has been in use for almost as long as there have been police officers tasked with upholding laws passed by legislators who have no idea of the impact of what they are writing.
Your point about wanting your 6 year old up front with you is a valid one. The truth is that some children mature faster and better than others. Many can behave themselves while riding there just fine. That does not reduce the hazard of being in that seat, it is still the least protected passenger space in the car. Today, with the airbag problems that we have, placing a small child in that seat is really putting that child at risk. As a responsible parent, it is up to each of us to consider the risks and do what we believe to be the best for our children.
I grew up always sitting in the back seat. It was a real treat for me to get into the front seat and that only began to happen when I was big enough to be considered a large child or adolescent. That was my parent's decision and was in keeping with what they knew of the safety factors of riding in a car. Although my ex-wife kept the car seat in the front with her, I always kept it in the back seat in the center. She claimed she could not see him unless he was up front with her. I never had a problem seeing him. If I raised my head slightly, I could see him in the rear view mirror of the car. I could easily reach behind me if he dropped or needed something, In fact, I found it easier to take care of him that way because I could place containers of food and drink on the seat beside me and easily reach anything he needed. I could simply hand it back to him without having to take my eyes off of the road for more than a few seconds. It also gave me the ability to know that he was in the safest location in which I could place him. If the car were struck from *any* direction, he was the farthest away from the impact I could hope for him to be. I continue to put his seat in the center of the rear seat to this day. And, I am happy to say, my ex-wife changed to that as well shortly after he turned 2 years old. She'll never admit it wasn't her original idea. 8^) So what? I don't care for credit, I just want my son to be safe.
So in conclusion. I think you and I are closer in our positions than we are our differences. I think you and I share the thoughts of Thomas Jefferson (?) who said, "He who is willing to give up essential liberty for comfort deserves neither." I am opposed to too much regulation. I am also opposed to harmful stupidity. The balancing act between the two is called politics; and politics is a real Bitch!
Koala