Karma isn't just random Internet points (in fact, that purpose of karma is pretty irrelevant and one's global "karma score" could just as well be removed without consequence.) The real point of voting is that it sorts comments, so that the dross of a conversation can fall below a certain line and you can stop reading the conversation before that line. Astroturfers negate the value of sorting, such that you have to read (or at least skim) the dross if you want to find the good comments.