I pretty much agree with oriongirl on hybrids, and wouldnt have them in my tank. in the case of the BP, should these fish ever have existed in the wild, they would be wiped out and extinct very quickley. Not so much seems to have been mentioned about the flowerhorn, but with over 2000 species of cichlids in the wild, why bother keeping them?
On the subject of selective breeding, agilis hit the nail on the head about wild guppies. I saw them in thailand, in water features, (of which there are many) outside shops and houses, to keep the mozzy larvae pop down. Very quickley its apparent that these fish arevibrant, full of life and beautiful, unlike the inbred zombie fish that frequent many LFS.
But Ive even heard that the wild caught betta spendens now has longer, flowing fins, than it did 30 years ago. Obviously the long finned 'domestic' variety has been rereleases to the wild over the years. It just shows that the results of selective breeding can be far reaching.
To put it simply, I think people shouldnt think whether they COULD, but rather whether they SHOULD.