another point that is rerely raised by the "they are all dogs" people - there are known and provable problems when certain breeds of dogs are crossbred that can seriously effect internal organs, fertility, etc of the offspring. Oddly enough - the same sort of problems that occur when crossbreeding divergent species of other animals and very similar to problems seen with crossbreeding some species of plant.

In fact - these days the "cannot breed" is more applicable to GENUS lines rather then SPECIES, and doesn't hold true even there. less so in animals then in plants.