I NEVER blanket....well, that is not totally true I guess, but I do not "routinely" blanket just because the temps get down to a certain point. We get plenty of 40 below weather here, and I do not blanket. The only time I will/have blanketed, is if a horse is obviously cold...ie: shivering. This happens sometimes when a horse moves here from a warm climate, even after her has been here a full year or more, and just doesn't seem to acclimate too well. This however, is very rare, and has only happened a couple of times over the years. I also have blanketed mares after foaling if they sweated a lot and are wet, just to keep them a little warmer, and only until the dry off. Basically, just to keep the chill off.
OOPS! Edited to add...Maryann reminded me that yes, I also do blanket newborn foals, but normally not for any longer than a few days, and even then, I try to get the blanket off during the day and blanket only at night, if temps are cooperative.