I voted that I dislike cross breeding of dogs. I do not think it should be illegal to do so.
What I do not like about it, even with the cav example, just putting titles on a dog, and doing some health screening, does not create a new breed or even the best of both worlds. Taking the get from this breeding, and taking the get from a similar breeding pair and breeding that, and down the line, does not make a new breed.
Most likely, you will have dogs that have a variety of the traits you wanted and the traits you do not want and are susceptable to all the health concerns for both breeds.
To start a breed of dogs properly, you would have to have a ton of people involved, and a massive amount of money and time invested. You would have to start with a vision -- a medium small dog specifically to hunt and kill spiders -- yeah, I would probably go for this.
First you would have to find dogs like Rosa that are good at killing spiders and research their make up. Maybe find five or six breeds of dogs that would be used in the mix. Then you would want each of their breed history, health history, etc, so that you would get specimens that would be excellent to start off with. And down the line, you still may find that you need to remove an entire line of dogs, and be willing to do that.
generations pass, with anal attention to every single breeding, what it produced, what were the results, etc etc, and then you decide you need to inject more terrier, or something that will make them more low key, or something that will give them more or less height or weight, and you add a little of something else in there.
Again, breed, breed, breed, evaluate, cut, remove, until you are producing something with enough regularity and with the origianal purpose that you can actually write a standard and stop allowing the addition of more purebred dogs in.
With all of these experiments and breeding pairs, enough so that you will not have to miserably inbreed for, where are all the puppies going that are not selected for breeding? Where do the washouts all go?
If there is actually a need, say for a bomb sniffing dog under eight inches, then I would approve of going ahead and trying to develop that. But for the most part, we have bird dogs for all occasions, hound dogs for all occasions, herding dogs for anything you want to do. And the list goes on and on. If you really cannot find a dog that covers what your needs are in all the breeds that are currently recognized by some kennel club, I guess you might have a case.
Most people do not have the resources or following to do it right though.