I believe that the breed mentioned was White Leghorn Bantams, which are very common, and it isn't very hard to find a strain that is diverse, and has all of the proper traits to create show winners, again and again if you know what your breeding for and what your doing. There are other breeds that are hard to find, and that the good ones are even harder to find, so if you want good ones you might have to add a thing or two to the genetics of the bird to get them to the standard. If you breed a bird to a bird just because it is the same "breed" to a point that it has no resemblance to the standard, that, in my opinion, is worse then adding another breed for one generation, to add the desired traits you want in your breed. You can see this being done in other species too, Guernseys crossing to Red Holsteins to improve their production and size, and I know I've seen graded goats for sale before, I just don't know much about them.
Haven't the master breeders on here been saying all along that you don't even need to mix strains- within a breed?
But, it's ok to cross breed with a different breed altogether.
You are not going to convince me. If heritage livestock breeders can improve/preserve their breeds without cross breeding, then poultry preservationists should be able to do the same.