There is a lot of evidence that hints at it. The reason I believe it though is simply the lack of other sensible options.
Griever has no decent explanation, not one that doesn't require huge theories being created anyway. There's a lot of foreshadowing too, and R=U fits very well when looking at Sorceresses going bad without knights, something to do with feathers, the theme of fate, a load of things that I've forgotten since it got banned.
It's never been proved correct, it just fills in holes, and I'd say is about 50% probable, I like the theory because FFVIII is too simple without it.
The other support is that it never gets any reasonable counter arguements. All the arguements I've heard either go directly against things said or shown in the game, or just go straight against common sense (how can Ultimecia die and Rinoa not for example, which goes against basic logic).
I know that doesn't explain the theory, I just thought I'd explain a biut about how previous debates go.
