User administration, subsiting ... Log on once and be able to visit the entire site, for instance, rather than requiring log-in to a third-party webmail package. Subsiting so that webmail could be appropriately placed in the site map (and permissioned etc). Proper navbars generated for webmail mounted in various places in the map. Integrated "look and feel" (OK, that's not a datamodel integration issue, but a consequence of using a single templating system).
Stuff like that.