I have thought about this off and on quite a bit, some day I might do it. The problem is that if they do breed, the young could pass off as one of the adults. You would know that it was a cross, but if you sold it to someone else, that other person might sell it without telling future buyers that it is a hybrid. Everyone knows that blood parrots are hybrid fish, but the danger comes in with less obvious fish, such as rainbow fish, killis, and Endler's.
Plenty of people got burned when greedy breeders would spawn guppies and Endler's together to maximize yields. Customers would buy these fish and keep them in the same tank as pure strain Endler's, and in no time these hybrids would be traded around unbeknown to hobbyists. Same with killis and rainbows. IMHO the issue isn't if you or I are conscientious hobbyists, the issue lies with buyers further down the chain. One bad apple or dishonest person could do a massive amounts of damage to the hobby, and they might even get away with it, long enough at least where the damage done is irreversible. I often wonder why people who don't want hybrids risk the chance by keeping different species in the same aquarium.
If I ever did spawn a hybrid strain, I would not feel comfortable selling the offspring at all. I would have no way of controlling what happens after the fish leave my hands, and I would feel very bad if the worst did happen, because I would be one of the people responsible and to blame. If I ever succeeded and had to many offspring, I would make plant fertilizer out of them or feed them to a predatory fish. Maybe I would trade them to hobbyists who I think are responsible, but I would be responsible if they distributed the hybrids around and the worst happened.
Long speech, I know, but there is a lot to consider if you or I go down this road. I don't think there is anything wrong with hybrids, but all hell can, and has, broken loose when they leave our tanks.