No one is debating whether or not puffy coats have the potential to contribute to accidents. What people (or at least me since I seem to be posting on this thread a lot) are saying is that there are times when there are real safety concerns with exposing your child to the cold, and that parents might want to weigh those concerns. More information, including statistical information, would help parents who have concerns about hypothermia and frostbite (which also kill children) make educated decision. You can believe that puffy coats are not optimal for kids in carseats, an also decide that in a specific situation they're the best available option.
For the record, I do things frequently with my child that have lead to the death of other children. I take steps to minimize those risks, but I don't eliminate them. Today we went skiing/snowboarding, despite the fact that I am aware that people have died doing that.
I once made the choice to allow my toddler to ride in a car with just a seatbelt. I'm sure there are people who would condemn me for that choice. However, in my circumstances I felt it was the best choice. We had gone out for an outing on the subway without the stroller. My son was at the age where he wanted to walk and I thought it would be easier to walk with him if I wasn't also pushing the stroller. Of course I didn't bring a carseat, and I didn't happen to bring a carrier either.
When we were about 10 miles from home, my son fell, hit his head hard on the corner of a wheelchair ramp, which triggered either a breath holding spell or a seizure. He then passed out. We went to the nearest E.R.. When they finally released us it was 1 in the morning, dark, freezing cold (which we weren't dressed for, the temp had fallen dramatically when it got dark). The hospital was in a sketchy neighborhood, as was my home, there had been a series of armed robberies on the street we'd need to walk on to get home. My mother, who has a carseat in her car was on vacation, and any other friends I could think of who might be able to come get me with carseats are single parents so they'd need to bring their children who would then be occupying the carseat. Oh, and I was throwing up non-stop from the stress. And my son was dependent on a feeding tube and a pump -- which was at home, which meant that staying a hotel or sleeping in the hospital waiting room until daylight was out of the question.
I figured my choices were
1) Take subway home -- risk hypothermia (maybe I could have begged a blanket from the E.R.) risk me dropping him or falling while throwing up in the metro, and risk being mugged or killed walking from the metro.
2) Spend the night in the waiting room or a hotel. Risk dehydration for my child. Or feed him by mouth and risk pneumonia.
3) Take a taxi home. Have him sit in a seatbelt. Risk an accident.
I decided that #3 was the best idea, because I had some idea of the statistical likelihood of each one. Information helped me make a good choice.
But in CPST world (at least on this board) it doesn't work that way. When someone asks what the OP asked (they asked how bad it was, not whether it's bad) they get told it's bad, and don't ask questions. Like they're two and can't be trusted to make their own decisions.