From what I've read, the whole purpose being showing a dog is to promote the breed, and to breed dogs who are perfect examples of that breed. So if you remove the ability to reproduce, you can then do nothing to forward the breed, so what's the point of saying "This - this is a perfect Golden Retreiver! This is the dog I want to mate mine with."

Still, I think they should just split every show into two - for the breeding dogs, and the non-breeding "just for the glory and fun of showing" dogs!