For us we love the fact that your own inbound email generates a notification. This helps confirm to the user that their message was posted, especially since there are many instances currently where posting fails silently. Right now, we can tell users if they don’t get an email notification, then their message didn’t post. With your suggestion there would be no real way for the user to determine if their message had posted without having to visit the website.
Plus most “old school” email listservs (mailman, LSI) send you a copy of the message you post to the list, so I would think users that you’re migrating over to Discourse would be more familiar with this “confirmation” style of posting.