I think that the final bosses should be powerful, but only as powerful as the story, or the game, allows them to be. For instance, I thought Jecht could have been much stronger. Sure it was hard without break damage limit and break HP limit, but when you get these abilities and level up so that you have 200 - 255 attack, you only need to hit jecht 3 times before he goes down.
Optional bosses should also be stronger than the final boss. Though it does seem wierd how something can be terrorising the world, while something stronger is lying dormant just waiting for your party to fight it. Like many have said, optional bosses are for those hardcore gamers how want more out of the game.