She's by far the character with the most influence on the plot, and along with Fang is the only reason the events occurred in the first place. Unlike Fang and Lightning, however, she has vital connections to all the important characters of the game.
"Lightning is the poster girl, that makes her the MC." You do realize that's Vanille, Fang and Ragnarok on the damn logo, right? http://images.wikia.com/finalfantasy/images/1/11/FFXIII_Logo_Art.jpg I have never heard an emptier argument, oh my gawd.
It's ridiculous to call this the "Lightning Saga", since it's only now that she has a game of her own.