Let me offer my two cents, as well as, opinions from other professionals in the horse industry. Ultimately the decision is up to you, but maybe this will help you understand why you should do it and if it is appropriate and necessary for your own horse...........
It's strange how the everyday practice of blanketing horses causes so much uncertainty in conscientious owners. Stranger still is the degree of passion blanketing arouses among horse caretakers.
To the question, "Must I blanket my horse?" the short answer is "no." The horse generates his own blanket--a haircoat that is long enough and thick enough to withstand the coldest days of winter. It's an adjustable covering that flattens against or elevates above the skin as the horse grows warmer or cooler.
"Hair is a great insulator, and it fluffs up to warm the horse," says Michael Foss, DVM. "Heat rising from the body warms the air, but that air doesn't go anywhere because it's trapped between the hairs."
As for the question, "Should I blanket my horse?" the answer could also be "no," but special circumstances make "maybe" or "definitely" the correct responses for certain classes of horses. Blanketing is necessary for competition horses and foxhunters who are routinely clipped during colder weather to maintain a sleek appearance, reduce sweating, shorten cooling-out time and speed drying after rigorous workouts. Aged horses whose appetites and digestion may not supply enough fuel to keep flesh on their bones and their internal "furnaces" stoked require shelter or blanketing during bad weather.
Relocated horses transported from a warm locale to a much colder climate often need additional covering for their first colder winter. Horses relocated before the autumn equinox have time to grow a woollier coat to match the colder weather, but even then they may not be sufficiently insulted for the new climate.
Added to these "must haves" are all the horses who are blanketed mostly for the owners' peace of mind and/or convenience (it's a lot easier to lift off a layer of mud caked onto a blanket than to curry it out of a winter coat).
If you choose to clothe your horse, the crucial decisions aren't the color and style of the "outfits" but your daily judgments about how much protection your horse needs and the best way to protect him from the irritations and hazards that accompany blanketing.
One really also must be careful that your blanket is not doing more harm than good for your horse. If he has a long winter hair coat and you blanket him/her with a blanket that has less insulation than his natural coat then you are actually reducing his ability to stay warm. The blanket will cause his hair to lay flat thus reducing it's ability to keep the heat in and warm his body.
A horses winter coat is the best insulation they can get by far. It was custom designed for them!
My opinion; unless your horse is older, very young or just doesn't get a good thick winter coat (because of being clipped, breed, location change, etc) DON'T waster your time and money on a blanket. Save you and your horse the aggravation and hassle.