A horse can only ingest so much hay. I could surround my horse with round bales and when it drops below 10 degrees he would still lose weight. He cannot eat enough to make up for that which he loses due to being cold. As he is a hard keeper that I struggle to keep weight on, oh yes I blanket him!
I see absolutely no GOOD reason to watch a horse stand around shivering. It probably won't harm or kill them but why should they be miserable? My kids own 50 jackets of all different weights so that they do not need to shiver or be miserable and I see my horse no differently.