"Definition of 'Species' from Saunders Veterinary Dictionary: 'A taxonomic category subordinate to a genus (or subgenus) and SUPERIOR to a subspecies or variety...'"
They mean 'superior' only in terms of nomenclature. Nothing to do with any genetic superiority. In othwer words nomenclature goes: genus, species, subspecies, with 'genus' being superior to 'species'. The idea certainly cannot be applied to the use of two species during the formation of a hybrid. Anyway, in agricutlural circles what on earth would be the reason for crossing any two species if one was genetically inferior?
"Humans already enjoy the longest lifespans of any mammal. How would inter-species crossing improve that?'
It's very doubtful that any humans could form hybrids with any living species. However it by no means follows that they were necessarily unable to form fertile hybrids with species now extinct. Hybrid vigour is not usually a product of between species hybrids. It occurrs within separate populations of the same species.
"And I certainly don't believe it's worth the trade-off in IQ points, just for a random chance of possible increased 'vigor'"
Exactly why farmers would not cross two species if both were not economically useful. It is the combination of useful characteristics they are after.
"In my OP, I was talking about ~inter-species hybridization with Humans (ie Human x Rhesus macaque or Human x Chimpanzee) not about simple outbreeding between different races of the same species".
Even chimpanzees are too remote from humans genetically to form hybrids. In general the time since separation for two species to be capable of forming hybrids is something in the order of a million years. Of course it's not specifically time dependent.
"Donkeys & horses aren't different races of the same species - they're distinctly different species, w/ different numbers of chromosomes)"
I keep trying to tell you that chromosome number is irrelevant. Obviously horses and donkeys have not been separated long enough to be unable to form hybrids.
"You breed Humans w/ lower apes, & you ~might get some 'hybrid vigor' out of it"
Extremely unlikely to be able to do it in the first place.
"Evidently, Neanderthals were the hybrid offspring of Humans x Rhesus monkeys; which then were subsequently ~backbred into the Human population"
Where on earth did you get that information from?
"Many gardeners prefer heirloom varieties of seeds over hybrids, for very good reasons (better flavor, better results from propagation of saved seeds"
Very true. But hybrids are used commercially because the production is greater with careful selection of the parent and grandparent species. For example hybrid hens lay far more eggs than do any 'heirloom varieties' of poultry.
"most people don't bother saving the seeds of hybrid crops, due to the unpredictable nature of their growth and development"
But that is the point of using hybrids. Their ancestry is selected to bring out the best points of each in the final cross. The F2 generation can throw up any combination of characteristics of the ancestral varieties.