I've not read that book, but I've spent hours upon hours discussing breeding among dog breeders. The bottom line is that everybody seems to have their own strong opinion, irregardless if it is backed up by science or on-hand
experience. Some breeders have crazy ideas that have very little to do with reality. I've talked with people who were very set against purebreds, but frankly, I couldn't see where their arguments held
water. They simply couldn't defend their viewpoint when the discussion got beyond the first 2 minutes. As you can see, I'm not against the concept of purebreds. By the way, neither is nature, if you've noticed.
Consistency is most easily achieved by using purebreds. That's part of the definition of purebred, the ability to reproduce traits with consistency. Thus if you have a pig breed noted for giving consistent quality carcasses, then you can expect to see a high percentage of good carcasses in the litters you produce when breeding purebreds. If you keep the best individuals for your breeding stock, then you
should be able to maintain that trait.
Carcass quality can often be achieved by crossing two known breeds, where it was previously learned that doing such a mating gave good carcass quality. Say for example, for years the farmer down the road has been mating Berkshire boars to Yorkshire sows and getting piglets that result in the ideal carcass for your needs. If you do the same thing, assuming that you choose good quality boars and sows, you'd also get the consistently good carcasses that you desired.
Once you start mating crossbreed animals, things can get a little fuzzy. A purebred bred to a crossbred will give somewhat predictable results, but there will be some variability that you can't control. You will see a higher percentage of off-types in the litters. But perhaps you are gaining the benefit of something else, such as increased litter size, that offsets the disadvantages. But once you start mating crossbreed to crossbreed, consistency goes out the door. While the farmer may see a tendency for a large herd, as a whole, to be ok when breeding crossbreed to crossbreed, on the individual level there is going to be wild variations. Thus a small farmer is not going to reap a benefit when breeding only a few litters of those cross-to-cross animals.