There may well be a reason it is retained. In my PhD work I found that really fast, slender legged dogs hyperextend the forefeet down to the stop pad when at a full gallop. In fast galloping dogs with small tight thumbs the claws wear with racing so that they do not need periodic trimming. I have had dogs who had had thumbs removed get injuries in the thumb stump region unless they were run with their pasterns taped. In tight footed breeds that gallop or dig the athletic animal has a use for the thumb and it should not be removed as a preventive measure.
Pastern injuries are as probably as dew claw injuries in the working animal.
Dogs that repeated tear their thumb claws need the thumb removed completely or they they need the claw removed but the little digital pad that accompanies the thumb claw brought into place over the end of the remaining thumb bones. To leave the thumb bones under the skin without protection on the projecting end is to set up a situation where abrasive injuries can occur when that part of the foot comes into contact with the ground during the gallop. These injuries are not really nasty but they can lame a dog on the day of a competitive event and cause the owner to have to forfit a tie, etc.