CONSIDERING THE FACT THAT MOST PEOPLE ALLOW EVERY SINGLE AEON SHE GETS. 3/4 OF THEM ARE OPTIONAL. most of her wins are due to aeons that should be illegal for her to use. Im far from being biased towards Yuna. If you allowed all her Aeons, then it would only be fair to allow characters optional boss forms(SUPER BOSS FORMS) or alternate forms from diffferent games. It would then be legal to allow secret super weapons and optional powers that you would not get if you completed the game normally.
1/2 are optional out of raw numbers, though three of them(Magus Sisters) are connected. 5/8 out of actual summon commands.
Superboss forms are a seperate issue, as are cameos. It's pretty reasonable to allow superboss forms from the original game-if you can come up with a time this really matters that actually isn't some optional area that makes no sense plotwise due to the fact that the original person is somewhere completely different, and hence it's debatable if it's actually them or just a random joke character, such as the Valkyrie Profile Seraphic Gate superbosses, then please, let me know. The one thing I can come up with, Monster Z, has the plot of being...one-time in ACF, which makes it an entirely different can of worms.
Cameos are even worse about being "them", since there's no concievable way the original characters could have gotten there. They're a joke with no plot continuity whatsoever.
Having said that, though, if you want to allow either, more power to you. The reasons I gave are why most don't.
As for optional weapons, why, yes, people do allow those if they're legal. Same with spells. Yuri gets hyped for Seraphic Radiance all the time, as Elfboy noted... He also has it as a major plot element in the first game. It's still a pain in the ass collosally easy to miss possible to miss permanently skill. People still allow it. 'cause it's, well, them. It makes them a more interesting dueller, by and large.
Plot Materia? Sure, allow those if you feel like it. None of them actually matter, except Restore's horribly shaky claim(Barret has a claim to it, so does Cloud. I'm not about to give it to both and both seem to have good claims to it.). People do allow initial Materia and have for a while.
Still, I'm not going to object if you find Yuna much more interesting without optional Aeons and ban them just for that reason; It's...biased and rather ridiculous since we have Godlikes that can still beat her, but whatever, I don't actually care. Just don't bawl others out for it, please.
The important thing is that there will be cake.
THE CAKE IS ALIVE.
YOU BASTARD YOU TRIED TO FEED PEOPLE BUNDT.
*Runs screaming.*