The new commenting system allows users to edit their comments, however if this functionality is available, I find it necessary for us to have control over which users/licenses/roles/groups etc. can edit their comments.
Let's assume that in your WF instance there is a legal/compliance-related workflow where conversations or updates carry high importance or sensitivity. Being able to edit a posted comment as you wish may cause confusion or disruption when a post containing vital information is suddenly changed - in such a situation it's better and safer just to post a new message to rectify the previous one. A big plus would also be an audit trail of edits made to comments.