First of all, let's look at the concept of "preserving": The show world was, from the very beginning, subject to romantic idealization of what a dog of a particular breed should be. Most of the breeds that became consolidated as a result of the standardization movement that started in the mid-19th century in the UK had never existed before that point, and certainly none of them whatsoever had been even remotely as uniform as those people envisioned/wished for/deluded themselves into (however you want to put it). What had existed were certain types, i.e. the famous sagaces, pugnaces and celeres the Romans used to classify hunting dogs as, which existed in some local variations that could most aptly be described as "landraces".
Then let's look about the value of having a breed around. Do dogs give a bleep about what breed they are? Hardly. Keeping a breed around is a concept that serves human interests, not the dogs, and when people talk about "preserving a breed", they must realize that they are not doing that as a favor to the dogs, but as a favor to other humans. Now, such interests can be entirely legitimate (domestication being a utilitarian concept), but let's not delude ourselves into thinking that the dogs care even the least bit about it.
Third, let's look at the philosophical concept behind dog showing (ignoring its rather unsavory roots in the eugenics movement): It is based on a fundamental fallacy, viz. the idea that you can theorize about how a dog is functional based on anatomical details. Show people come up with elaborate theories how such-and-such detail serves the dog's function in a very specific way, which sound logical and are internally consistent -- their problem is just that they are not usually tested in the real world on dogs doing work besides trotting around the ring for a few seconds.
Summarily put: Dog showing puts the cart before the horse. A show person thinks that a dog has a certain angulation of its hind legs and would therefore be a good working dog. A stock man observes a good working dog and breeds it to another such dog, and this may have the side effect of resulting in a certain rear angulation, even though the latter was never consciously selected for. In showing, function is thought to follow form. In reality, form is merely a side effect of selection for function.