While I do see the pros for this feature, I disagree that this is something which should be imposed - I think it should be a configurable option. Let me just mention two instances here on moodle.org when I found it advisable to delete my posts.
- Somebody who was having a problem with the uploadusers script posted an attachment with sensitive information while seeking help with the problem. After I replied and solved the problem, we both deleted our posts to protect this information.
- Today I offered a solution for somebody, then realized that there was a much better solution. While I could have left the old post in the forum and then offered the better solution in another post, I thought it was better to delete the old post in case somebody hastily searching for such information later might be misled by the old post.