I am using session resumption with large resumption time - 24 hours - http://xmpp.org/extensions/xep-0198.html . I have a doubt about muc, if I am in group and my client rejoin, do I need to leave and rejoin the group to get the new messages. For non muc stanzas, they are kept in offline storage and delivered as soon as I reconnect. But I am not sure about muc.
If it is not so the case I don't think there is any advantage of being online - except to fetch occupants list. Again I am not sure if I will get occupant joined event if my client was offline.