I managed a private farm in extreme Northern Maine for a few years and these were Arabs. Some were blanketed...those in training and those who would be shown in early spring. The retirees and youngsters were not, nor were the broodmares.
The key to weather-proofing is to not do much grooming. Keep that dirt in the coat, just a quick, light once-over to satisfy your own sense of esthetics.....get the shavings off, hay and tangles out of manes and tails, and fluff up any hair matted from sweat or rain or snow. Run your hands over ever square milimeter daily to check for problems, but put down the curry comb and definitely don't vacuum. It is hard at first, but I had arabs outside with run-ins who would stand in the rain and snow rather than come in.
I can only think of a couple times that a mare misjudged and came in shivering. I threw a polar-fleece and a cotton sheet (works better than a winter blanket for this purpose because it lets the moisture out while keeping the horse warm, and good quality polar fleece wicks.) after scraping, of course.
A full belly of hay keeps them warm through the fermentation in the gut. So feed more stemmy hay is your guy gets too fat on lots of hay.
The younger horses need a bit more protection. Also, if you do not have the good fortune of a run-in situation where the horse has a choice, just watch the attitude. Is he grazing, dozing comfortably, or standing miserably by the gate with his head down and ears back and tail kinda tucked? Bet not!