What appears to be known for certain is that there are ultra corn snakes with known, lost, or otherwise unknown grey rat snake in them and it is also known that this is also a highly controversial subject as well. The point being is that hybrid genes are already mixed into a great many corn snakes and the decision was made that this mutation would be treated like pure corn as I have come to understand it as it affected too many corn snakes and too many breeders. Which is another reason why keeping good records is of utmost importance if you are concerned with purity. But, what this shows to me is that purity is an arbitrary subjective term if we just look at these facts alone.
Never mind the fact that what we consider to be a species is another man made and artificial designation. Species, however we look at them are constantly evolving. Species are commonly accepted as being able to interbreed and produce fertile offspring capable of continuing the lineage. For me, I understand those that like "pure species" or "pure local" specimens. However, unless your working in conservation, catch and release, etc.... the corn snakes one breeds regardless of "purity" or "local" are not breeding or being selected for in any natural way.
A dog being released back into the wild to replace its lost wolf population or corn snake breed for our own purposes and tastes and released back into the wild makes about the same sense to me. The purpose of breeding for a hobby such as this is to often create something new or novel. We are seeking to alter the corn snake in much the same manner as we have altered dogs from their wild wolf counterparts. To think of corn snakes in the hobby in other way simply does not make sense to me. So, when I think of hybrid, pure, or unpure... these are all artificial constructs that while they do serve their purpose for a conservationist... serve little purpose to someone who just wants a cool looking corn snake be it hybrid or otherwise as snakes bred for a hobby group are not under the same selection pressures as their wild counterparts.