The insulation a healthy horse already owns
A horse's winter coat is not a passive blanket. Each hair can be raised or flattened by a tiny muscle, trapping a layer of still air against the skin when the coat stands up and shedding heat when it lies flat, while natural grease in the coat throws off a good deal of rain before it ever reaches the skin. Horses also run a higher tolerance for cold than their owners do — a dry, still, frosty day that feels bitter to a person in a yard coat is well within a healthy horse's comfort, provided he is fed enough forage to fuel the furnace. Rugging interferes with this system by flattening the coat, which is why the decision deserves more thought than habit gives it.
The factors that genuinely tip the decision
Work through the individual horse rather than the yard's custom. A clip removes the coat wholesale, so a clipped horse has signed away his insulation and needs it replaced by rugs matched to the clip's extent. Age and condition pull hard the other way: the elderly horse, the one carrying too little flesh, the horse who shivers visibly or stands tucked and miserable in bad weather all benefit from help their younger, rounder fieldmate does not need. Breed and type matter — a finely coated, thin-skinned horse feels a wet wind that a hairy native ignores — and so does the field itself, because a horse with decent shelter, hedge or field shelter can manage weather that would defeat one standing on an exposed hill.
Weather that changes the answer
The combination that strips heat from a horse is wind driving rain, because the coat is soaked through and then chilled as fast as it can be warmed; this is the weather in which even hardy types are glad of a rug or a shelter. Cold, dry and still is a different proposition entirely, and horses across cold climates live out unrugged through far worse than a frosty week. Judge by the horse in the weather, not by your own hands at the gate: a relaxed horse grazing with a level back is coping, whatever the thermometer says, while shivering, a tucked-up posture and turning the tail to the wind are a horse asking for help. Never rug a horse who is already wet and cold in the hope the rug will warm him — dry him first, or the rug traps the chill against him.
If you rug: weights, fit and daily checks
Choose the lightest rug that does the job, because over-rugging has its own costs: a horse who sweats under an unnecessarily heavy rug loses condition, rubs and ends up damper than the weather would have made him. Fit matters as much as weight — the rug should sit clear of the withers, allow the shoulders to move without chafing, and stay put without straps cranked tight. Most importantly, a rug hides the horse: weight loss, weight gain, rain scald and rubs all develop invisibly underneath, so lift or remove rugs regularly, run a hand over the horse, and reassess the decision as the season turns. Making that rug check and change part of the routine recorded in KinsLe's Care Teams means the horse under the rug gets seen every day, whichever member of the team is doing the yard.