Hey Alexey - I can see where this may be useful but also could potentially be bad as well. I know my scenario above was a bit extreme nor do I think any of my members would do that. 

I do think it would be best to connect to the membership levels just in case. For example, all users in talks should be able to edit their own messages. Maybe Talk owners can delete the messages. But the 'Edit All' feature should be left for Admins and Moderators. Just my opinion