In NE Nevada where it gets -20 in the winter, I never blanketed. Those horses were just fine with bifg fluffly coats. Even it it snowed and they had two inches built up on their backs, which meant they were insulated and not losing heat. They knew how to deal with the conditions. They knew how to break the ice in the river and dig through the snow to find the grass. If the snow got a big hard crust they got fed hay just like the cows.
Take them same horses and move them to NE Texas. Different ball game. Even though the winters may not be -20 and snowing. The rain and wind is what kills them, they are miserable! They are turned out in cow pasture with no run in shed(just like at home in NV), when the grass is no good I always have a round bale out in the feeder so they can munch on it at their leisure. But when it rains sideways and they get wet and cold and get to shivering I will admit to drying them off with a towel and blanketing until the rain stops.
The key to blanketing is the weight of the blanket, in my opinion. I understand the thought about a blanket laying the hair down and allowing it to fluff and trap the heat but if you blanket with the appropriate weight even if the hair is laid down from the blanket the insulation should keep the horse warm. For example, throwing a waterproof sheet on when it is raining, windy and 33 degrees....he most likely is still going to be cold, miserable and worse off than if he didn't have the sheet at all.
It is the rain that does for them. Our TB broodmares never got rugged up, they did have a shelter that they would use - I know we don't get the extreme cold in the UK but it was always the wet weather that did for them. We had some youngsters living out one year when it rained every day for weeks and weeks and we ended up blanketing them because we just physically couldn't get enough food in them to stop them from losing weight.
The whole hair fluffing up will keep them warm is something of a myth disproved by biological science knowledge. The hair stands up when its triggered by the cold feeling on the skin in an effort to keep the cold air away from it - it happens with humans too we just done have enough hair for it to be effective and that same fact applies to many horse breeds that man has interfered with through breeding or just didn't evolve in an area that got that cold. A prehistoric breed like the Exmoor is going to cope fine provided it has a food source but a fine coated TB is never going to do so well or will need way more food.
I think it comes down to common sense. You keep an eye on your individual horses, if they are regularly uncomfortable/unhappy, are not adjusting, and/or losing weight, then a blanket is a good idea.
Yeah, my horses have days here and there where they aren't all that happy with the weather. Either it got cold too quickly or too warm quickly and their coats aren't where they need to be, but a day or two isn't going to hurt them (probably is beneficial in terms of helping their body adjust to the changing weather pattern) and as long as they have adequate shelter/food I see no reason to jump on blanketing/clipping. Again, if I had a horse that wasn't adjusting I WOULD blanket, however I've yet to own one that didn't. I don't think anyone is saying don't blanket a horse that needs it, just a healthy horse is generally just fine in most circumstances. Especially if they have good shelter and feed.
Frankly I spend more time miserably cold and/or hot during the year than my horses do by far.