While I understand that these people are trying to produce a dog with a specific purpose in mind, that purpose is, for all intents and purposes, a game, hobby, or recreational activity. I mean, we're not talking about working farm dogs, military dogs, police dogs, service dogs or other dogs that in some way benefit society. Can we really justify producing more mixed breed dogs for the sole purpose of playing games while there are millions of these same dogs available in shelters? I appreciate the good intentions but when you are mixing two very different types of dogs together you have no idea what you are going to get. What if they don't turn out like you planned? What if they don't excel at said sport? Then what happens? You don't get a black and white dog by breeding a black dog to a white dog-it's just not that simple. And not to belittle flyball but we need a specific breed of dog to jump 4 hurdles and fetch a ball from a box?
There is no guarantee that your $1000 purebred dog, your $1000 "well-bred" mixed breed dog, your purebred rescue, or your mixed breed shelter puppy is going to perform up to your expectations, not get injured, or be healthy. You can sometimes tip the odds in your favor with a responsibly bred purebred. But only sometimes. I have seem some very expensive dogs with bad injuries or illnesses. In the end you are better off to seek the dog that has the qualities you are looking for, regardless of where is comes from (bad breeders excluded of course!) and in the price range you can afford. There is no need to try and create one out of two very different dogs and hope for the best. If health concerns factor in many shelters and rescue would be happy to have the testing you want done perfomed. Even if you have to pay for it, you are likely going to get a better deal on your "Border Jack" from the shelter than from a "breeder."
And they can harp on genetics all they want as far as health concerns go, but genetic disease are tricky, and can come back to haunt you. There is a recessive gene in Chinese Crested dogs that causes a fatal, progressive brain disease. There is a recessive gene in Kerry Blue Terriers that causes a fatal, progressive brain disease. Do you know how they determined that this is the same gene? They bred a carrier of each breed together. This could happen in other breeds unknowingly. Aren't we better off to focus on eliminating diseases in breeds we know about instead of accidently creating more problems?
By and large most dogs today are pets. Even "working" sport dogs. And there are hundreds of breeds available to choose from. If none of those are what you want then there are literally millions of unique mixes available for adoption. And beyond this we often tout dog sports like agility or flyball as something any dog can do-so what does it say when all of sudden we decide that no current dog on the planet is good enough and everyone needs to have a dog that looks and acts specifically like "dog x" to compete effectively? At this time, my opinion is that we don't need to create more breeds of dogs. Obviously some will debate that, but they can start their own blog!