A knowledgable fish supplier will provide you with fingerlings that originated from multiple males and females to provide diversity. Also as a matter of practicality, production ponds wouldn't have just two parents because there would be a risk that one or both wouldn't survive to reproduce.
Adding fish from new sources would only be worth the effort if you suspected that all of your pond's fish came from a very few parent fish. Personally I like the idea of adding a little diversity, although I have absolutely no scientific data to support my hunch. It make me feel good, though.
