When I make a comment on another article it's usually because there's something wrong on the article. I never comment just to say something like "Great hub", I feel I can do a lot more by hitting the tweet or like button (and do that instead).
Oh but I have denied comments that provided a different perspective on some of my articles. Mostly because the said perspective was that of an insane person. My medicinal herbs hub, for example, takes a completely scientific approach to herbalism.
I don't want comments on there arguing about how all natural medicine is PERFECTLY SAFE...someone might actually ingest a lethal amount of one of these herbs because of those idiots.