I went through a phase of beating this drum loudly (about 25 years ago), coming out of a breed that had a very small gene pool (Doberman Pinschers--most American Dobermans can trace back to about 8 original dogs brought out of Germany before the war) By scientific definition, this breed, and many others with equally small or smaller gene pools should have died off by now due to the effects of "inbreeding depression". However this is not the case, they continue along, and while the breed certainly has it;s share of genetic problems, it continues to function and even flourish. There are many many other breeds in the same boat, and some with an even smaller gene pool.
There is a group of lions in Africa that have been much studied, they live in a huge valley created by an extinct volcano and are genetically cut off from the rest of the lion population. They are what we would call heavily linebred, with quite a bit of inbreeding thrown in--all by Mother Nature--and by "scientific" standards they should not exist anymore, let alone continue to dominate their little corner of the world. Nature has not seen fit to have them decide to breed to leopards. Cheetahs are in the same boat, and by genetic science should be dying off of all sorts of genetic defects; this has not happened with them either.
I have also worked with dogs in a professional capacity (as a vet nurse and a professional groomer) that I don't buy the theory of hybrid vigor ("mutts are healthier") I've seen many mixed breeds, coming from many generations of mixed breeds, with problems that would be considered genetic in a purebred.
The problem is not with the practice of linebreeding, inbreeding or outcrossing in themselves; They are valuable and necessary tools; it's where the focus of those practices lie. Is it to make money? Is it just to produce bench or field winners?
We are fortunate in cockers that we do have a diverse gene pool, what with partis, solids and workers, they represent 3 distinct different pedigrees (and in no way do I support splitting them into separate breeds!) We can weave outcrosses in our pedigrees without having to go out of the breed, and we are certainly not a breed that is in such trouble that it is necessary to go out of the breed to bring in "new fresh blood". And where would you go? As has been stated before, springers have genetic problems in common with cockers, plus a few of their own that would inevitably come along for the ride. And in strictest genetic terms, any of the spaniels would not be a true outcross, as they were all the same breed till the late 1800s, with smaller puppies used for heavy cover and wookcock (hence the cocker) and larger ones designated for springing birds and game (springers). And it's only been about 60 years since the breed that we know split in half--into English and American Cockers (due to an internal dispute about type and heads).
Combine our diverse gene pool with the wonderful testing available to us now through Optigen, (and I'm sure more new breakthroughs in the future) and we have a good amount of tools available to us to keep our breed strong and healthy!