Yeah, but the basis for E=U was that they used the same powers. If you then claim that the theory could work because maybe she lost her powers and they no longer shared abilities.... what's the point?

R=U is the only theory about Ultimecia that I've seen make any kind of sense.

Joel
QUOTE]2. Rinoa witnesses Squall kill Ultimecia at the end of the game, if Ultimecia wen't through all of that trouble to plan to possess people and try to take of there world with here powers then if she were Rinoa then she definetly wouldn't make the same mistakes twice and remebering all of the stuff that Squall and his friends did to thwart her plans then hereby not failing in her plans.[/QUOTE]

Fate is a huge element of the game. Even if she DID remember then she wouldn't be able to change anything because then she wouldn't see herself do it in the past and so couldn't change it.
Time travel is fairly shaky in VIII but I'm pretty sure the cycle of Sorceresses confirms that at least that version of time travel and cause and effect holds true.