You might be right, but I usually still don't care about it. Topics like that usually have a higher bar for thoughtful comments, but the ones that manage to get over that bar are still visible and discussed. Astroturfers are annoying, but then again karma is just random Internet points; what matters to me more is if a comment manages to start a discussion involving other HNers posting interesting insights.
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.
Well, I guess I don't trust greying-out that much or am just curious, but I tend to skim the heavily down-voted comments anyway. Every now and then, one can find a pearl of wisdom in them.
