Roy, I think it is mostly a lack of training knowledge on behalf of the handler.
In the hog hunting realm, we have to worry about our dogs "trashing" on cattle or other desired livestock. But there are quite a few hog hunters who use their dogs as cattle dogs by day and hog dogs by night. So that kinda blows the "one job" theory out of the water.
In fact in the "real roping" thread I posted, several cowboys sicced their cattle dogs onto a deer track and bayed the wounded deer then went on to herd cattle the rest of the day