I do it, and I plan to do it even more in the future, so I can hardly be against it. I do feel like it needs to be done carefully, same as purebred breedings, but I dont feel like its any different in that its better or worse, or more or less care must be taken, and it may even be better because >low COI
I would absolutely buy from people who do sport or pet mixes, provided they also passed my basic standards of breeders
we DO need a better **** name for them than "purposebred mixes" its too mouthy
Edited to add:
I am VERY firmly on the side that Companionship is absolutely a working task, and that a dog needs to be bred for these specialized skills in the same way a herding or pointing dog is. Meshing seamlessly into a world designed by and for a species of an entirely different genus is a very unnatural and specialized requirement. Like SpaceMutt said above, being amiable, moderately smart, confident and completely unfazed by literally EVERYTHING, easygoing in nature, but at the same time loving and friendly, without being overly enthusiastic, and also willing to be modivated, without being too drivey, all of these things are incredibly specific and specialized traits. and the fact that in a world designed for, and by, humans, there is nearly zero wiggle room for "annoying" or deviant traits, it means that these highly specialized and specific traits need a near 100% genetically-ensured (word i cant think of) rate. Oh and the dog also needs to be physically tough and healthy, because ordinary working people dont have time for chronic dog ailements. You DONT get these things by chance, and very few current purebreds are up to snuff, as we are just barely out of the agricultural revolution and most breeds retain pesky traces of working drives and abilities.
the world is becoming more and more industrialized, and there is less and less sympathy for deviancy and less time for dogs that require humans rearrange their entire lives to suit. hardcore working or sport people that dont mind doing this are the minority and denying it doesnt make it go away. more and more in the future, the pet dog is going to dominate, and they need to be GOOD pets, which means selective breeding. and if that also means crossbreeding, then so be it.
sorry for the novel. feel strongly, i do.