Pat, you're wrong; our cows produce nothing but the most protein-rich cream all winter long
Bent, yes it definitely has positive effects on production, and very few negative effects on anything.
A little more milk pr cow, and generally higher milk quality than what the average "indoors" dairy farms can manage. We're far above average milk quality at the moment, and that's reflected in what we get paid.
Also fewer health problems and much stronger animals since our cows "exercise" in fresh air in hilly terrain all year round. Means more well-adjusted and happier cows, and fewer veterinary-visits.
Another advantage is that the animals take care of themselves between meals/milking, which reduce the workload on us humans to a minimum most days and also lower the overall cost.
On the negative side is that certain accidents can happen more easily out there in the woods and in-part steep terrain, than on a flat stable-floor. Have had a couple of serious injuries from falls off cliffs in the last 10-15 years, one of them fatal.
Have also had to treat a number of more serious cuts and bruises, and did lose one cow to that last year. Healthy, strong, animals heal well though, so usually some first aid is enough.
Sometimes a cow may also hide a little too well and in difficult terrain when calving out in the open, so finding and getting the calf home isn't always easy. Usually the cow brings her calf home after a day or two though, and since cows are good mothers and take extremely good care of their newborn calves - much better than we humans can manage, there's usually no harm in waiting for her till she's at the gate with her calf.
So, all in all: positive, but strangely enough: Norwegian farmers need special permission to keep dairy cows outdoors all year round, and few Norwegian farmers seek such a permission.