I belong to the same club as Roy (GBSC) and I think the issue is not so much what the policy should be (pretty obvious that one or two of the duty crew must stay until all are accounted for), but rather that the club just hasn't articulated the need for that. On any given day most of the duty crew are not x-country pilots, they are students, juniors, or entry-level pilots, with the exception of the duty field officer in charge. So, let the crew go home and make the rule simple, "the duty officer, or a qualified designee, stays until the last ship is accounted for."