You, and other users both in this thread and in similar ones before it, are only looking at one side of the equation.
The problem is only partially too many bad comments, it's also too few good comments. Your proposal is targeted at lowering the amount of bad comments, but it might do so at the expense of the good ones.
Most of the users that have a high karma count have so because they always have something insightful to say (mechanical_fish (http://news.ycombinator.com/user?id=mechanical_fish) is a good example), and their incentive for posting interesting stuff is marginalized by this. Almost all people, whethere they'll admit it or not, are incentivized by other peoples approval, eg. karma, and downplaying their contributions will make them more prone to not submitting great comments.