Honestly, if you don't live somewhere where you have consistent below freezing temperatures...why take the risk? There are some great fleece, thin, coats that are very warm and do not put a child at risk.
I've got 8 kids, up to 2-3 in car seats at a time, and it's really not that hard or a big deal to remove bulky coats. We always carry a blanket or two, but even in below freezing temps, they rarely needed them because of the heater in the car. When getting out they'd unbuckle and pop their coats on. Again, not that hard.
Now a child can wear their coats if you do not have to adjust their straps. Meaning the straps are the same when they are in their seats with no coat. I have one that can do that cause she is so skinny.
I can understand parents who choose to leave the coats on in area's that are always freezing. An accident that knocks the parents out or God forbid kills them could lead to hypothermia for a young child if help isn't right around the corner. It's a judgement call that the parents have to make, but the rest of the population? It just seems lazy to not take the coat off.