Cawkazn, some breeds are good for crossing. That's all there is to it. All the stuff about individual dogs, working and show bloodlines, health selection and everything else is implied, but it boils down to the fact that some families produce better than others and that certain breeds within them are of better use for mixing than perpetuating their own pure form. You can take two crappy dogs of two proper breeds and produce a "bottom" litter of their mutts, but you can take those same lousy mutts and work and test the living daylights out of them and breed them until all the health issues and whatever else genetic ugliness rises up to the top, then cull even harder and select for genuine improvement until you have a bona fide elite worker with stable genetics to which you can forever refer as a cross of "A" and "B" ruined breeds. Or you can only use the best examples of whatever "ruined" breeds thinking that puts you ahead of the curve, by whichever criteria.