From The New York Times comes an article on Why Scientists Love to Study Dogs (and Often Ignore Cats). The bottom line:
[Elaine Ostrander, at the National Institutes of Health] wrote, “dogs have undergone this really striking bottleneck during domestication,” in which a few ancestral wolves gave rise to all domestic dogs. Later on Victorians produced many breeds that have even narrower bottlenecks, with much inbreeding.
Domestication, she said, has “happened in an amazingly short period of time and we don’t understand all the genetics associated with it. It remains one of the most interesting and challenging questions in biology.”
Some dogs suffer from behavioral problems that look similar to human problems like Obsessive Compulsive Disorder. Those similarities, Dr. Ostrander said, provide “a great avenue for learning more about ourselves.”
In short, inbreeding means that more genetic problems regularly pop up, and by comparing the genes of inbred dogs with non-inbred dogs, scientists can see where the beast has failed.