A piece of paper with photos of the sire and dam would also come in handy for those looking to select for top genetic specimens of a given morph. For example, a really nice looking pastel with photographic "proof" of really nice looking parents would be worth more to me than a really nice looking pastel from mystery parents.
... Though Foschi raises a good point -- anything can be forged.Without a DNA-verified registry, it's still all basically just an honor code on paper.