Well then people who show a de-sexed bitch/dog other than in veteran in my opinion can stay in a class of their own.
I don't know why you'd bother.
Also along the lines of what Occy was saying, I have a saying, keep the best one for show and breed from the second best. But the point is at some stage both dogs/bitches would have been at some point, in the ring.
Others due to an accident, knocked out a tooth, or damaged ligaments will stay for the purposes of breeding.
If you are really interested but still don't understand I suggest you read up on outbreeding, linebreeding and inbreeding and see the reasons behind why some dogs are shown, some aren't but nearly all are entire.