A post here from USDA on the use of sheep to reduce tillage in an organic farming setup. The idea seems to be to control weeds by grazing sheep on land for a year between row crops.
My comment is, as with other organic rotations like using alfalfa in a rotation, it's fine if you have a market/use for the product. Sheep herds have declined over the decades as we turned away from wool suits and mutton. The knitters of the world can absorb only so much wool from small sheep farms (which doesn't mean the prices of skeins of yarn are low). In the old days of horses and dairy grazing you had use for fields of grass; in the days of tractors and barn-housed cows you don't.
Farming like most any human industry is more complex than it looks from the outside.