I live in Southern Md where our winters aren't terribly cold but will
get down into the 20's some. If I had a horse that lived outdoors I
would not blanket unless they were outside my back door and I didn't
work away from home. Why?
1) Fall days can get warm enough that an insulated blanket will soak
your horse in sweat but the nights turn cold. Not healthy.
2) Rain will soak thru most blankets advertised as water
proof/resistant after a couple of hours unless you;ve spent big bucks
on the blankets with the inner membranes such as the Rambo's.
3) Even with leg straps a blanket can be shifted, twisted or tangled
about the legs. Horses and blankets end up the worse for wear.
If your horse is checked a couple times a day and will have his
blankets rotated if too hot, too wet, torn, etc. it can be done.
Otherwise, I'd suggest leaving them unclipped or if far enough south
(like in this area) you can clip out the neck and chest early and skip
blanketing. Enough hair will come back that they usually are fine
without blankets. The unclipped horse will get wet hair but a good
cooler (I prefer polar fleece over wool) will dry them off.
Personally, I blanket. My horse is on stall board with daily turn out.
She is checked frequently each day and blankets can be swapped as
needed. I'm not going to do more than a trace clip on my new fine
coated mare and will use an extra light weight blanket that is basicly
a nylon sheet with a polar fleece liner. This seems to dry the horse
back off without having to swap the blankets around. I notice there
are a couple of manufacturer's selling blankets that are basicly three
layers. I made up my own for a lot less.