Well, I guess I'm a bit of a rebel, but I don't approve of breeding outside the standard, or deliberate crossbreeds, for sport. For legitimate work, sure. But not for sport. Sports are a game, and while it's not unreasonable to consider these abilities in breeding, I can't condone breeding with that as the sole goal.
My standpoint comes as a Stafford owner/occasional breeder. I love this breed, and I love to compete in agility. I could breed dogs who were better suited to agility, but then they wouldn't really be Staffords, would they? I might as well just get a BC or a Sheltie if what I want is an agility dog. We have different breeds for a reason.
And the fact is, the Staffords that are bred specifically as "sport" dogs tend to be less than sound. There may be sport breeders in other breeds who pay some attention to structure, and not just drive, but I don't see it in this breed.
My own dogs are bred partly for their sporting abilities, but I also consider whether they meet the breed standard. Not whether they can win in the ring, as that doesn't always have to do with meeting breed standard. But they do need to have the characteristics of the breed, and I do get outside people (who know what they are talking about) to evaluate my dogs, rather than risk becoming kennel blind.
If someone really can't find what they want within an existing breed, there are many great mixed breeds available who would be excellent sport dogs.
I'm not going to run around dissing people who do breed solely for sport, because I'm not dictator of the world. But it's not something I approve of.