I find myself commenting under a lot of your posts, generally without a whole lot of additional content aside from restating parts of the piece and saying I agree with them or am interested by them. Does that make me a part of the problem? I'm not nessecarily doing it for upvotes, I find that if i restate something and write it myself I can remember things better, which then helps me apply it to my life. I guess in a way, it doesn't further discussion, but at least I'm not cloning other comments.
Sometimes I do have things to add to the discussion, and at that point I do so, but I don't always have something to add. Does that mean I should stop commenting in those cases? I like to tell people when I like their work, because as a writer myself I like it when others do the same for me.
It depends on each person and generally I know who is doing it for votes and who is not. Familiar faces come in and talk someties, sometimes, quote and agree, someties just say, thanks or cool. With a relationship I can upvote all of them as I know where they are coming from. An unfamiliar face that comes in and says 'cool' likely didn't read it at all.