Before everything else, this is NOT meant to be a flame thread.
So to ask. What's the worth in defeating Penance?
You acquire a Ribbon accessory. But by the time Penance is gone, you've probably defeated all the powerful bosses (excluding Braska's Final Aeon) in the game e.g. Dark Aeons and Omega Weapon hence there will no longer be any adversary to use the Ribbon with.
You also acquire a couple of Dark Matter. But by that time, you've acquired all the Celestial Weapons thus you'll no longer need them for customization.
Master Spheres will also be acquired. But by the time you've kicked Penance's ass, all the characters have completed the Sphere Grid rendering the Master Spheres useless.
Ultimately, there will be no extra ending for defeating Penance, not even an extra scene. after the battle, it will just shift back to the Airship, the characters acting like you never went thru a hellish battle with the boss with 12 million HP, the highest boss's HP in the history of Final Fantasy.
So to ask again, what's the worth in defeating Penance?