Here's our take on this very sensitive subject with many horse owners. We encourage you to read the entire post before you pass judgement on us!
Blankets are for people. Yep, we just said it. Horses in the wild don't get extra insulation - they have their own. Should they not be able to keep warm they will do any number of things instinctively to ensure they survive. If your horse lives outside, has never been completely body clipped, or resembles a Woolly Mammoth he won't need a blanket. Provide him a place to get out of the wind and let nature take over.
Blankets are for Horses. Contradiction? Not really. Consider that you're horse probably doesn't fit into the "wild" category anymore. You have the horse who shows, winters somewhere warm, lives in a barn, or any number of other domesticated situations that have taught the horse to be dependent on the Human. You need all the extra insulation you can get for your Horse. Get yourself a blanket bin and get ready to fill it up!
Here's our basic guideline for the average Horse and how to dress appropriately for the weather.
T-shirt weather No Blanket (temps in the 70's+)
Light Coat/ Sweatshirt Light Sheet (temps in the 50's, no sun)
Sweatshirt/ Fleece Baker or Woven Sheet (think early Spring/late Fall)
Mid-weight Jacket Light Fill Blanket (think early Spring/Late Fall)
Mid-Weight Jacket w/ Layers Mid Fill Blanket (temps in the 30's - 40's)
Winter Jacket Heavy Blanket (temps between 10 - 30 degrees)
Winter Jacket + Long Underwear Heavy Blanket + Fleece Liner (think sub-zero)
This is a guide - bottom line is as a rider, owner, professional, or equine enthusiast you know your horse better than anyone else. Know what keeps him comfortable. Sometimes it's a little trial and error, but if you're paying attention the error margins should be minimal. Bottom line is we support both methods of thinking by understanding where each is important. For the horse that lives outdoors year-round blankets really aren't a necessity; For horses that are pampered and stalled blankets are usually not an optional accessory.