When I got my horse, I was told he would always need blanketed in the winter because he doesn't grow a good winter coat and he will shiver too much and lose too much weight and he loves his blanket and blah blah blah...
He looks pretty fluffy to me!
I've never had trouble with his weight in the winter. I don't ride as much so he spends more time at the round bale. No blanket necessary.
If a horse is a really hard keeper, or if they have medical problems, or if they are just really old, yes I would blanket them with a really nice, heavy blanket. They need the extra help to stay warm and keep weight on.
If a horse is young and healthy and isn't being clipped for shows, I don't see the point of blanketing them. Healthy horses usually do a pretty good job of growing a thick, warm coat on their own.
Blanketing can actually make a horse colder by pushing their hair down flat. It's like taking the insulation out of your walls and wrapping your house in a tarp instead. Just let them grow that teddy bear coat and keep it clean, they will be plenty warm!