I consider the main point of retraction to be a reconsideration of the value of a comment - that is, it should be used to say "This is, on further consideration, wrong, or without value" while leaving the reasoning for why it was wrong (or without value) in the first place intact, so other people don't commit the same mistake. This is so regardless of whether the karma value of the comment is positive or negative.

Karma values are, in general, a useful gauge, but not an ultimate gauge, of the usefulness of a comment; they are in the end the consensus view of a post or comment, which may or may not be "correct".