But FF3 was just as vague! Think back to FF3 - sometime in the past, Warriors of Darkness arose because the Light became too strong and they needed to tip back the balance. But HOW they did that is never stated! What actions does a Warrior of Darkness take to weaken the Light? What quests did they do? What enemies did they fight? It's implied that what the Warriors of Darkness did was good and necessary, because imbalance is Bad, but what they actually DID to restore that balance is vague.
It's EASY to portray Light's fight against Darkness, because you can give it all the trappings of standard Good vs Evil (even though the game TELLS us that Light is not necessarily Good and Darkness is not necessarily Evil). You have plenty of cackling villains to defeat, monsters to slay, dark temples to explore, and so on and so forth. Things start to fall apart, though, when you try to imagine it from the other point of view - when the representatives of Darkness are the heroes. What quality makes a cackling Light villain different from a cackling Darkness villain? How is a Light monster different from a Darkness monster? The writers never say.
All FF3 (and this game, as well) says is that Imbalance Is Bad, which is an easy, if pithy, notion to get behind, but exactly WHAT we're balancing is never spelled out. Light and Darkness are vague, meaningless terms on their own, and the writers can decide however they like as to just what things count as Light and what things count as Darkness.
Some posters have pointed out that Light and Darkness COULD represent Order and Chaos. Warriors of Light arise when society is collapsing, and Warriors of Darkness arise to shake things up when the world becomes too rigid and stagnant. Not a bad notion, but the game never makes that clear, so: vagueness. Even if we assume it's true, though, it's tough to look at the state of the world in game and say, "Boy, this world is so stagnant and orderly that we're on the verge of a Flood of Light!" Every city we've visited in the game seems like it could collapse into civil war if we did even a little bit of "shaking up".
Kugane has it's revolutionaries and deeply corrupt nobles. Ishgard has had a theological upheaval and the common folk are starting to wonder why they toe the line when the Houses aren't as "noble" as they were taught. Limsa Lominsa needs an iron-fisted leader just to barely keep its population of pirates from running amok. Gridania is forced to levy severe and seemingly arbitrary punishments against people at the whims of the Elementals, and is engaged in what could be called a small-scale war with poachers. Garlemand's current regime was explicitly created for the purpose of sowing chaos. Ala Mhigo and Doma are only just beginning to pick up the pieces to form stable societies after being liberated. Only Ul'dah could be said to be a bastion of Order, thanks to the self-serving desires of the Syndicate to maintain the status quo - and even that city has no shortage of malcontents. None of this paints a picture of a world so overwhelmed by order and stagnation that the forces of chaos need to step up to maintain the balance!