I have to say I agree with this.. I have a Jack Russell and she is the sweetest dog ever, loves everyone, and we have two cats. As nice as she is, and as much as she licks the dickens out of one of the cats (is too scared of the other) she does go to Kennel when we are not home. I have researched and am thoroughly convinced if I don't do that, I will come home to a dead cat someday. It is just her nature. She has killed 14 snakes in her year and 1/2 with us, and is very interested in the neighbor's chickens. We have taken measures to keep those dumb chickens safe (though the dog was here first, and is predatory by nature, but instead of building the coop in the side yard, it is right in my dog's view...).
So even with a very loving dog, there are genetic attributes toward doing predatory things... but to say that Jack Russells (or german shepherds etc..) shouldn't be bred is going a little far, isn't it?
It is just like saying that bad parents that raise bullies shouldn't be breeding. Though in principle I might agree, it isn't really realistic.