First of all the two species do not occupy the same ranges in the wild and therefore no hybridisation is known naturally.
Secondly, there are genetic differences between the chromosomes on the two species - one difference being in the sex chromosome. Unfortunately I cannot recall the exact details of the differences regarding the number of chromosomes that differ and their individual differences, but I do recall that one particular chromosome is longer in one species than the other.
Although I can see the logic applied to gaining a pure animal from hybrids, it assumes a simplicity in the differences between the two species. In reality this does not happen - probably because there is more than one chromosome which differs between the two species, therefore reducing the chances that all the Campbells chromosomes and genes will appear in a single hybrid offspring with none of the Winter White chromosomes or genes.
Hybridisation may also cause chromosomes to be modified - ie where the chromosome is of two different lengths a hybrid may contain a chromosome of middle length. Obviously by mating back to the species with the longer length chromosome would then increase the length of chromosome in the offspring each time but it would require infinite matings to get back to the original length in the pure species. There are some breeders in Holland who claim to have 98% Winter White blood hybrids bred from taking an original hybrid and repeatedly mating generations back to pure Winter Whites - these are still clearly hybrid in appearance. These hybrids do tend to have more characteristics of the Winter White than the Campbells but even with 98% Winter White blood they are still quite clearly hybrids in appearance. This would suggest that changes made in hybridisation are difficult, if not impossible, to undo.
Often the hybrids produced from breeding Campbells and Winter Whites together are infertile although there are those that are fertile. However, these fertile hybrids when bred together usually go on to produce infertile offspring so it is not possible to sustain an indefinite line of hybrids.
Mating a hybrid back to one species may well improve fertility, but the fact remains that if the production of hybrids were to become widespread we could see a real fertility problem in the future.
Other factors that should be born in mind are:
1. In some cases, breeding a Winter White female to a Campbells male has led to the female being unable to give birth and resulting in the loss of babies and female. This is due to the Campbells being larger than the Winter White and the female having problems giving birth to the unusually large babies. The breeding of Campbells female to Winter White male does not present the same problems.
2. The incentive to breed hybrids is often the wide range of colours in the Campbells and limted range of colours in the Winter Whites. Often breeders mistakenly think that if they breed a coloured Campbells to a Winter White they will obtain a Winter White in a that colour. In fact what is produced is a hybrid in that colour and not a Winter White in that colour.
3. If a totally new colour should emerge in a hybrid this colour could not be recognised in either species as it would not be a colour mutation of the true Campbells or true Winter White. The colour could only ever be recognised in the hybrids. Without the breeding of the hybrids this mutation could have occurred in the pure species and been recognised so in effect a colour would have been "lost". If the breeding of hybrids became extensive this could reduce the number of pure Winter Whites and Campbells in circulation and therefore being bred, and this then decreases the chances of natural mutations occurring in each species. Therefore in an effort to increase the colours available in Winter Whites breeders are actually decreasing the chances of such colours occurring and surviving in the purespecies.
4. Given the birth problems and reduced fertility in hybrids if the breeding of hybrids were to become widespread resulting in a decline in pure species available, we could see a time come when the majority of russian hamsters in captivity are unable to breed succesfully.
Given the above, I cannot see that the breeding of hybrids can be in the interests of the hamsters, nor the interests of the hamster fancier. In short there is no reason to deliberately breed hybrids. I accept that it can occur by accident with ignorant owners, pet shops, etc and this cannot be helped. However, I do not believe that there is any benefit to be derived from the deliberate production of hybrids.