Just to add a point I don't think anyone has made on this string, let's consider what happens if people try to breed for a very uncommon trait. In my breed, the gray color is uncommon. When these dogs show up, there's usually nothing special wrong with them, but they are penalized in the show ring so there is no reason to try to breed for that color. As long as the dogs just show up when that is how the genetics happen to fall, the color will probably not be connected with a health problem, and the same may be true of the long-haired Rotti.
But these dogs with the anomaly are a VERY small gene pool. If people were to start deliberately breeding to try to get more of that trait, chances are very high they would wind up breeding dogs with very serious problems who would suffer and whose owners would wind up broken hearted. The gene pool is just too small. And there is no reason to do this. Gray dogs of my breed lose one of the most important traits in the breed, the red color. They look as much like some kind of Husky as they do like their own breed.
Rotties are rough and ready-to-go dogs, which long coat would detract from. Long coats require a lot of extra grooming time, cause the dog to heat up more when working (which is what Rotties were bred for: work), collect stickers and things that interfere with the dog's movement, and breeding for that coat would likely increase the incidence of health problems in the resulting long-coated dogs.
We need to just place these unusual dogs in appropriate homes to enjoy them when they happen naturally, and not try to borrow trouble by breeding for a trait that does not have an advantage and would surely create suffering for dogs and people.
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Kathy Diamond Davis, author, "Therapy Dogs: Training Your Dog to Reach Others," 2nd edition, and the free Canine Behavior Series articles at http://www.veterinarypartner.com/Content.plx?P=SRC&S=1&SourceID=47