OMG, CESQUE! So what if Odin alone has a different backstory every time? The games have totally different stories every time! Different worlds! In fact, in case you haven't noticed, the summons change their appearances in every game. If you think about it, maybe the summons aren't EXACTLY the same summons from previous games; they just have the same name and attributes as in the previous games!
Oh, and by the way, fighting Tiamat again isn't that big of a deal! You are just whining about names! WOuld you rather have Tiamat, a totally cool dragon with an equally cool name, be called "Bob the Dragon", the lamest name of all? Look at Zelda! They change the world of Hyrule in each one! And Link always "FINDS" the master sword in each one. Funny, I dont see you complaining about that, huh?