The point isn't moot because people can still buy a PS3 that is backwards compatible.
Just why the hell would a Final Fantasy fanboy go over to the Xbox 360 when all it can do is play Final Fantasy XI and XIII?
Common sense and logic dictates that when a Final Fantasy fanboy is guaranteed to play I, II, IV, V, VI, VII, VIII, IX, Tactics, and Vagrant Story on the PS3 and there is a PS3 model that allows them to play X, X-2, XI, XII, and Kingdom Hearts I and II, they're going to go to the PS3.
In the end, Final Fantasy XIII will end up shifting more PS3 hardware for two reasons:
1. It is still exclusive to the PS3 in Japan and Final Fantasy sells a boatload in Japan. As a matter in fact, the PSP is doing phenominally in Japan thanks to Crisis Core.
2. Final Fantasy fanboys will still go to the PS3 because they can play almost the entire core series plus several related games to the series and Final Fantasy Versus XIII is still a PS3 exclusive.