Why do breeders test for brucella?
Because it is a very serious disease, highly contageous, can be passed on to humans and makes breeding dogs sterile. So if your breeding outside of your own kennel, or your dogs ever go off your property, or another dog ever comes in, there is a risk. It kills newborn puppies and sometimes adult dogs. A lot of dogs have been euthanized with as there is no known cure and the risk to humans is present. IMO, since it can be easily transfered from dog to dog, breeders who do not test for it are asking for problems.
There are in house tests available, so that could be why its not a common test at a lab, but there have been outbreaks of it in the south and MI / IN / WI in 2007. I dont know of any more recent documented problems, but have heard rumors that make me uncomfortable enough to never skip this test.