I have to agree with jon on the issue. My parents had a dairy in northern sweden, where, due to the climatic conditions (deep snow a lot of the year) they kept the animals tethered.
I work in the dairy industry in Australia, and also have my own property and I can say, both from research and my own experiences that animals who are free to express their natural behaviour are happier, healthier, more productive and less inclined to vice like behaviour. Not to mention the animal welfare aspect; there is a big move by consumers away from the more'conventional' factory farming approach and back toward the more extensive and holistic systems so even from a marketing point of view you are on to a winner with 'free-range' animals if you lead the way.
There is ofcourse another important factor to consider which is that outdoor animals are less work intensive! You dont have to bring feed to them (if they have access to pasture at all times of the year anyway), you dont have to clean up after them and provide them with bedding and the amount of shedding and infrastructure required is minimal compared to housing an entire herd indoors!
Just a few thoughts...hope is of some help!