I blanket my horses, even when I boarded them in a stall and they were only out during the daytime at the warmest hours.
Currently, all 4 of my horses live outside. They have shelters & trees, and buddies - although they don't really cuddle. I don't like the horses to get super duper thick winter coats, so here is my blanketing regime:
When the sky gets gray & there is no sun & the temps do not get above 50-60 during day and lower than45- 50 at night they wear a lightweight sheet with no fill. Especially if it is raining, it keeps them far cleaner. If it is sunny, the sheets come off, as they can get too hot and sweaty under them, even in cool temps, if they are standing in the sunlight. Lightweight sheets usually get put on in late october around here. But last year, it was still high 60 in November.
When the temp doesn't get higher than 30-40 during the day, and isn't lower than 20 at night, they wear their medium weight blankets, with about 150-200 grams of fill. They are usually in these from about November through January.
When the temp doesn't get higher than 20-30 during the day and drops even lower at night, they are in their heavyweight turnouts with about 300 grams fill, and I have neck sheets, too for some of them.
2 winters ago, the temp dropped to 25 below zero with a windchill of 40 below one night. I put both the medium weight & heavy weight on the horse that night and checked on him around midnight, as I was fearful for his ears getting frost bite.
However, this is just me. Mostly I do it out of habit and desire to keep the horses clean and slightly less full of hair. When the temp dropped sooooo low 2 winters ago, my horses' buddies withstood those temps with no blankets at all, and all the horses were just fine.
I know dozens of horses that do not wear blankets ever, and they survive the winter perfectly fine.
As long as the horse is healthy, fat, and has plenty of hay to eat, they will stay warm. Corn & corn oil generate warmth by taking longer to burn (making the body work to burn food will create warmth). So, I always add a little corn to their feed in the winter time, but again, I know horses that don't get this, and still do fine.
I guess when it comes down to it, blanketing your horse truly comes down to your opinion. I tell ya what, it gets costly having 3 blankets for all 4 horses, not including the anti-sweat sheets & fly sheet for summer day time turnout...... They get ripped, they need repairs, you need to get the professionals to wash them, as they won't fit in most standard washers and you can't sneak them in to the laundromat usually, they pile up in the tack room and take up a lot of space.
Blankets are a pain in the butt, and they aren't necessarily needed, especially if your horse is in good weight and healthy and is receiving the right types of feed to generate warmth throughout the night (and day).
It seems like every fall, one of the equine magazines usually runs some big long article that is usually entitled something like "to blanket or not to blanket". And the morale of all the stories is, it really comes down to owner preference in healthy horses.