IIRC, the current cron rebuilds the sitemap from scratch each time it runs. So running it daily would require a lot more work than it really needs to, especially on large forums.
IMO, a cron should run daily that adds any new entries (filtering by last cron time should be sufficient) to an "updated" sitemap which would be mentioned in the sitemap index. Then once a month or something, have a cron that rebuilds the entire sitemap to cleanup deleted, moved content, and such and merges the "updates" into the main sitemap.