To me the dividing line is more a question of motivation. Some things with affiliate links are written because the person genuinely wants to say something about the subject, and then adds in affiliate links to get a little money without (as you note) diminishing the user experience at all. But other things are written solely as scaffolding for the affiliate links, content-farm style. The latter I think generally add noise, while the former are unobjectionable. It's not always clear which is which, though it's clearer at the extremes.