People can build a high total karma just by posting a lot,

So what?

because one thousand comments with average karma of 1 provide more total karma than e.g. twenty comments with 20 karma each. But which of those two would you prefer as a reader, assuming that your goal is not to procrastinate on LW for hours a day?

When I prefer the latter, I use stuff like Top Comments Today/This Week/whatever, setting my preferences to “Display 10 comments by default” and sorting comments by “Top”, etc. The presence of lots of comments at +1 doesn't bother me that much. (Also, just because a comment is at +20 doesn't always mean it's something terribly interesting to read -- it could be someone stating that they've donated to SIAI, a “rationality quote”, etc.)

Every comment written has a cost -- the time people spend reading that comment. So a neutral comment (not helpful, not harmful) has a slightly negative value, if we could measure that precisely. One such comment does not make big harm. Hundred such comments, daily, from different users... that's a different thing. Each comment should pay the price of time it takes to read it, or be downvoted.

That applies more to several-paragraph comments than to one-sentence ones.