I see what you're getting at now, sorry, missed the point initially. Just my guess here but perhaps it's a preparatory step for you getting a source of AC power back but only after more than 5 minutes when the right IRS has dropped out, say an APU start attempt is unsuccessful at high altitude so you try again lower down and AC power becomes available, then the FO's displays are powered once again but attitude information will be lost from it's normal source, using the IRS transfer switch guarantees usable information quickly. As for trying to start the APU at high altitude, it might be justified if you happen to be very close to an airport and you'd bet the mortgage you'll be on the ground within half an hour. Someone else may be able to shed some more intelligent light on the inclusion of the IRS transfer switch step in the NNC though?