I have never blanketed my horses, and I live in the northeast US. Our winters get snowy, wet, and fairly cold (down to like -15 degrees F), and my horses stay outside without shelter, and are always fine. We used to keep them in a pasture with a run-in shelter available, but they never, ever used it, even when I put the hay in there. So now they just live out in the weather, and they don't mind at all! Sometimes I'm amazed at what they can stand without freezing!
Three of my six horses are Thoroughbreds, and they happen to get the thickest, fluffiest winter coats out of all of them! I don't think that the breed matters...if you leave a horse to deal with the weather and let his natural warming capabilities kick in (and don't keep him inside, blanket him, or otherwise baby him), he will be much better off. Horses are made to adapt to weather, and protecting them from it only screws up their natural body functions.
Courage is being scared to death but saddling up anyway. ~John Wayne