I'd be **bleep** off if Sephiroth was in and he wasn't playable.
If anyone ever really paid attention while playing FF VII, you'd realise that you never actually fought Sephiroth until the end. The entity that appeared to be Sephiroth throughout the game was actually Jenova.
What? There's never any ambiguity that what you fight throughout the game is Jenova. You can look at it's name while you're fighting it. Also there's no enemy that's supposed to be Sephiroth or looks like Sephiroth until the final boss.