It would be useful at least for proper search (Discourse breaks browser search and replacement it provides is horrible), downloadable archive of public postings would allow this.
Though I have not needed it so far, as forum is not old enough for me to try finding something.
Trigger for posting this thread was Proposal:Require proposal announcements to be made on the new forum instead of the mailing list - OpenStreetMap Wiki (search for “self-hosted with OSM account log in”)