I personally haven't downvoted Tim, although I now feel like I ought to have, simply because it felt like bad etiquette to downvote someone else while in a sustained argument with them, even if you feel like they're engaging in bad reasoning. I should probably be more liberal with my downvotes in future than I have been.

If a person is making positive contributions to the board with some regularity though, I think it's worth having them around even if they are also frequently making negative contributions. At least the karma system gives people a mechanism to filter posts so that they can ignore the less worthwhile ones if they so choose.