Bungfai: I assume that the flight in question is not required to be `augmented` by your local CAA so, normally the captain can leave the flight deck for `physiological reasons`
ie. toilet breaks, even with a CPL rated F/O. However................. if his wish were to spend extended times off the flight deck leaving 2 CPL F/Os in charge, who may not have had `Relief Commander` type of training and for there then to be an incident like a depressurisation .......... I think at the subsequent board of enquiry the captain might have a lot of trouble explaining his actions!
Usually augmented crew schedules are derived by the airlines in consultation with their relevant CAAs. The `extra` crewmember would usually have had some additional training.