Editing should be allowed for an hour or so in order to fix spelling mistakes, reformat text for easier flow, or even as a recovery after pressing "post" by accident. But long-term deleting and editing will hurt the content. This, for example, is visible on reddit where older threads sometimes have these big piles of deleted comments. Instead maybe I would try something new:
1) Ability to post anonymous comments
2) Instead of deleting a comment a user can choose to make it anonymous so that it is no longer associated with his/her account