You' re right, every FF has used this system - that doesn' t mean it' s acceptable to the palette of all gamers.
the bulk of the game sees you levelling up/advancing your characters in tandem with a story. As you level up, you' re doing it in context of the story, so you feel there' s a purpose to it.
But if you want to take on Sin at full strength, you have to do the levelling up without any real story behind it. This isn' t a fantastic incentive if you ask me.
The last 10-20 hours are spent advancing your characters wihtout any kind of character or plot progression. That' s why I feel the FF series is flawed, and has been flawed for far too long.
And not every RPG adheres to this system. Hell, I went through Skies of Arcadia from start to finish purely in context of the story and I finished it, without having to resort to endless searching and levelling up.
Don' t get me wrong, I enjoy FF for the most part, I just think it falls down in the last 10-20 hours.