A very touchy subject for many horse owners.
In a very natural environment: plenty of acreage to roam, including natural wind breaks and shelter, I would say that one does not need to blanket. With that being said... Not many horses in domestication have a truly natural environment for roaming and foraging. Most are in small paddocks, stalls, and under 10 acres of pasture.
There is also the work schedules of the owners/partners to said equine that have them on regulated feedings and conditioning. There isn't free foraging and roaming needed for the horse to fuel its internal fire to keep them warm.
Then there is the region of the world in which you live. I am in Texas and truly cold cold nights only happen for a very short period of time. I have not always lived in Texas and I have not always blanketed my horse, nor will I always blanket. When I lived in Indiana I never blanketed my horses. They were used to the cold weather, their coats came in thick and full and fuzzy. However here in Texas things are not all that consistent. This year my horse hardly has any winter coat, we have had another extremely hot and long summer. It is now December and we have had 80+ degree days and then suddenly it will drop into the 30's. This is when I blanket. The horses have no chance to adjust for this much of a variance and I believe I am reducing my risk of colic and illness.
Is there a "right" and "wrong" answer to this question? I don't think so. I think every horse owner has to look at their own horse environment and each horse individually and make the best decision for their equine needs.