A simliar question was posed on our pilot group e mail recently. If the aircraft is full and the resting crew can't get back to the front due to a fire or structural failure, where do they sit for landing?
On some of our flights where the CRC is available and first class is full the captain rests in the CRC at the rear. In the case that he can't get back to the front he becomes excluded from the decision making process and taking his required position for the rest of the flight. In the much more likely case of a depressurisation or uncontrollable fire his input to the 'master plan' following the IAs may be a long time coming.