The other category to consider are those domainers who hold websites which are _not_ typically covered in ads; they might just have a notice on them saying "contact firm X for details." In many cases, I have even written such outfits saying, "is XYZ.com for sale?" and been told no, it isn't, or something like that. I have seen this for some domains which are more high-end, more generic useful words and less just pure spam.

This said, it's still a disgusting waste of internet real estate.