Like Skyblade pointed out, Ultimecia's attempts to kill herself and Squall etc. are usually explained away by assuming that Ultimecia, despite being Rinoa, was so far removed from Rinoa mentally that they had essentially different identities and Ultimecia would have no recollection of her past. Of course this makes one wonder why bother with the theory to begin with, but there you are.

The most damning evidence against the theory is the complete lack of good evidence in favour it, both in the game and in the Ultimania.