I actually think it's easier to sell a grade horse because most of them are under priced and a lot of registered horses [at least in my area] are extremely over priced.
As to the question at hand, I doubt any reasonable person has anything against grade horses - if they have been responsibly and intelligently bred. Many crosses are better at the job they do than a purebred would be. Remember, that all breeds arose as a cross or combination of established breeds. A "junk" grade is another matter of course, but a lot of people breed "junk" purebreds too, and they are no better than "junk" grades.
Using grades as breeding stock is, of course, a different matter. Breeding horses with unknown ancestry is not generally prudent because the product is unpredictable.
IMO anyone that has an issue with grades is no different than a person that is prejudiced against Arabs, Quarterhorses, Appys, Morgans, TB's, or any other breed. Once a horse is born, the measure of its usefulness is its dispostion and ability. Whether it is registered or not is irrelevant other than when it comes to money for the reasons I mentioned above, and of course being qualified to participate in breed shows.
I have never quite understaood the prejudice against grades, and can only attribute it to foolish snobbiness. If you breed a champion stallion to a champion mare and produce a registered champion foal, is that foal any better quality than if the breeder had failed to submit a stallion report and the foal couldn't be registered, so is a grade? Or if a grade soundly beats a registered horse in a race or competition, is it of lesser quality than the registered horse?