Agreed, i giggle when i hear of people bundling up for 40 degrees! here in NY clipped ponies get blankets when the low is less than 40, and bundled up hardcore if less than 10. In his woollies, my guy went all winter with a reasonably thin blanket. If it got in the negatives he got his hood and a thicker one, but between 0 and. 30, my little man was quite fine. Never had a sick or shivering horse and we hit -15 routinely it seemed this winter. Obviously old and young are different. Remember, if you are cold, a horse still has a horse hair coat on! They might be hot!
I'd prefer a horse a little cold than sweated up under a blanket in the cold. If its rainy, I think a lot of them enjoy it, especially after a super hot spell. Otherwise, they have a shed.
However, I have a tough one sheet, which is fairly waterproof. I use it for shows and traveling in the rain to keep my white horse white! I coated it with some camp dry, makes it extra tough!