Uh, because they're still the same person? Just because Ultimecia is completely different than Rinoa, doesn't mean she wasn't still born as Rinoa.

Yes, they 'might as well' be different characters in that sense, but they aren't in the R=U theory. Same person, different personality.

Oh, and GF usage isn't the main argument used for Rinoas change of personality, simply because it's too speculative whether or not she used GFs after the game. It's her insanity at Squalls death that's the main argument.