I think that the key here is to understand that there is a difference between "careless interbreeding" and the skill that it takes to develop a stable, attractive ornamental line of fish. It's the difference between "I have a tank of mbuna and one of the females is holding"...and a specific line of flowerhorn (or fancy discus or "swordtail"), for example. Indiscriminate breeding is bad, whether with different species or (allegedly) "pure" ones.
I think that there is a key difference, too, between what goes on in the confines of the home aquarium...and what goes on in the wild. The risk of flowehorns taking over the native streams of, say, Florida from the oscars, Hemichromis, Plecos, peacock bass, etc, etc, etc. and other non-native (but pure!) species is next to nothing. The risk of confusing a flowerhorn with anything other than a Trimac is also next to nothing. And if people want a verifiably "pure" Trimac, they should buy one from a source that can provide the provenance of the fish. Same with convicts or anything else.
Matt