As far as I'm concerned, Jecht is the final boss. I would say the other two battles are more just plot wrap up in battle form. It would have been really annoying if you beat Jecht and the lost to your Aeons, and had to go through the whole sequence again. I guess they could have just given you the oportunity to save, but then people would accidentaly save over their game, not knowing they couldn't go back. I have never had a problem with the way they did the end of the game.