That's the question isn't it?
Instead of defending the light together, each Union is trying to dominate the others, even though each Union's Foreteller warns the player about the "Seekers of darkness who are disguised as Guardians of light."
Whatever they're trying to achieve they use artificially created keyblades wielders (the players) to reach this goal. Artificial because Chirishii clearly states that the player's keyblade (Starlight) is incomplete and can only be used by drawing power from the cards (called "fragements" by him/her/it). You can also cleary see that the keyblade is incomplete when the keychain is changed to the known KH/KHBBS keyblades.
So the good guys haven't show up yet. At least that's my theory at the moment but because they're weren't more substantial story missions yet it could be outright wrong.
Perhaps WE aren't even playing as the good guys.
Think about it. The only goal given to us was to protect the light (lux), which we get from defeating Heartless. However, as players, we are instead obsessed with gathering lux for ourselves, for self gain. Ask yourself, why are you desperately collecting as much lux as you can? To protect it? Or to reap the benefits?
By collecting lux, you can gain power. You earn rewards at certain milestone points, which give you an advantage. You aim to collect more lux than others, desiring to have a high enough rank to earn exclusive benefits, which will allow you to maintain your position, and defeat others. You are given the ultimate reward for placing well: more powerful cards.
It is this quest power that fuels us to continue, not any noble desire to help. We are selfish, looking to hog as much light for ourselves in a desperate attempt to gain more power through better cards. In a sense, we are giving in to the darkness, probably without realizing it. We think we are Guardians of Light, but instead, we're all Seekers of Darkness. We are simply unaware of this fact.
This game creates an explanation of HOW the Keyblade War began. One individual, probably someone similar in nature to Master Xehanort, threatened the light. People desperately tried to protect it, trying to do good, but failing to realize that they were fueling the war and giving in to the darkness, allowing it to spread. The fighters in the war weren't evil. They were ordinary people trying to do good, but falling victim to human nature.
