My horses go out in day, in at night 365 days/yr.
To blanket or not to blanket is a judgement call based on age and health issues, IMHO
My horses are ages 17, 18, 25, and 26+.
If the temps are in the 30's or lower, it's cold/windy and no sun, they now go out in waterproof, lined blankets.
I have never blanketed horses in my life but, I started that routine in 2011.
The 17, 25, & 26+ all have arthritis, two of them have metabolic issues, the 17 yo's winter coat looks like most horses' summer coat and he's always been that way.
I feel they've reached a point in their lives, along with their health issues, that blankets are necessary for certain types of weather to prevent them from burning calories just to keep warm. Plus I've got enough vet bills as it is