It's a good idea, already on my to-do list, but it requires other changes as well. If the person who made the rating can delete it as they can today, then they can also delete the response, which isn't good. My current tentative thinking on how it should work is:
 - Raters can retract ratings, but not totally delete them.
 - Raters can add a short addendum to their ratings, and this can be freely edited, but the original rating can't be edited. (The point is to prevent conversations via rating edits. A rating should be one particular issue, and the response should be a response to this issue. Not 100% sure that this is necessary, though.)
 - Additional limits on how often you can leave ratings may be needed.