I work my guys very hard in the winter. Because of the cold weather and the groomed snowmobiles trails I travel fast and far in the winter. I clip by guy and bury him under 2 warm blankets. With a clipped coat and temperatures well below freezing he can maintain a good lope for hours . I don't worry about breathing, only about how hot he is getting.
If I worked him like that with a full winter coat he would be soaked in no time and drying him would be next to impossible.
So with a clipped horse I can bring him into the barn almost dry, throw a good blanket on him and send him out.
Horses with full winter coats just can not work to their potential