I have Parrot fish and I like them.
I also have a mixed mbuna tank and I am sure there is plenty of mixed breeding going on. While these fish will never leave my house, I don't cull the fry that survive. While I agree that these "mutt" hybrids should not be sold and I would not even give them away, I did purchase my Parrots and have grown to like how they look. My wife loves them and that is really enought for me to like them.
I think there is some sort of line between everything goes (like fish mutulation and dying and even tattoing to nothing goes (like PETA). I read PETA stand on fish keeping and some parts made sense. While I did not agree with all of it, I can see their point.
These fish are not in nature, we have chose to keep them in our homes in a prison made of glass for our enjoyment. While we should not harm them, if in captivity they breed differently than in the wild, what is the harm. As long as these fish are not mixed into the rest of the fish trade, then I really see no harm. I guess people could just not mix certain breeds or try to keep all males (or all females), but I think accidents will still happen. Killing off the fry from these "unatural" pairs is worse than letting them live.
Too me there are larger concerns that face this hobby. Things like the sale of large fish to people with small tanks, the mixing of fish that will attempt to kill each other, the transportation of fish before and after the sale and poorly maintained fish tanks. I also think their should be a ban on those freaky clams that open up with an air bubble (lol).
Like I said, I have Parrot fish and I like them.