Let's get the terminology straight first. "Intergrades" are not the product of crossing subspecies. Intergrades are individuals that exemplify a phenomenon within population ecology, called intergradation, where two subspecies are separated by a population with characteristics from both. What you get when you cross subspecies or species (or genera for that matter), is hybrids. Both in the wild and in captivity. The only "big" difference between subspecies and species, is that subspecies have diverged later, but they've still diverged.

I know this hobby uses the term "intergrade" for subspecies crosses, but quite frankly, this hobby is to a high degree comprised of people who are short-sighted, narcissistic and unscrupulous and who use whatever euphemism they can to make it seem like what they're doing deliberately isn't as bad as it actually is, consciously or unconsciously. And I do hope I've just mortally offended many of them.

Now, in regards to hybrids, I see no valid excuse for producing them and they only cause trouble for people down the road. Worse yet, they have the potential to spread to wild populations, they may have developmental defects and they may make a population susceptible to diseases that it was not susceptible to before (incidentally, that's one reason why translocating animals to new populations is frowned upon as highly irresponsible these days). Above all, people don't want them muddying the waters and being passed off as something they're not. Personally, I don't want man-made hybrids out there in captivity or in the wild contaminating the gene pool and if I had my way, each and every one of them would be euthanized or sterilized, and that includes everything from Felis silvestris x Prionailurus bengalensis to Canis lupus lupus x C. l. familiaris to Thamnophis sirtalis sirtalis x T. s. parietalis to Populus tremula x P. tremuloides.