Personally, I don't even really /like/ the idea; sure, it's one way to explain things away, but I find it dull. It basically says 'everything ever is about Squall and Rinoa; there are no other motivating factors in the universe.'

I much prefer the idea that Ultimecia is a totally separate person, with her own motivations and goals that she simply felt no need to disclose. It opens a lot of doors, and doesn't twist Rinoa's character beyond recognition.