Platties and sword will cross breed and the offspring will breed. They can be very attractive fish and they can be very ugly fish. I actually don't think my LFS
has had a true platty in a long time. They all look like the crossbreeds I have had before I knew better. Survival rate is lower, they don't get along with most other fish, IME, and are nothing like the original species. They seem to change alot and act a differently.
Why, who knows but I wouldn't recommend it just because it pollutes the already hard to find true strain gene pool in the LFS