I agree it would lead to less interaction, but the interaction lost would only be downvotes being used as a disagree button. No one is going to get harassed for downvoting a bot posting an ad or someone just completely off-topic, that which the downvote is suppose to be used for. In your scenario you point out that the comments you seek out have so many upvotes-downvotes because it's controversial, not that it doesn't add to the discussion.