The Avatar is supposed to be a being who brings balance to the spiritual and physical world, correct? Well, how exactly is the Avatar going to bring true balance to the world if the Avatar isn't balanced itself ("it" because it can be male or female)? Balance = just that, "balance" – neither good or evil... Do you see where I'm going with this? Yes... the Avatar is only comprised of the "good" spirit of the Avatarverse: Raava. To bring true balance and tranquility to the world, the Avatar has to be balanced morally as well, not just good, but evil as well, thus making it a "true" being of balance. In a nutshell: for the Avatar to be able to bring true balance to the world when a shift in balance occurs, it has to be a being comprised of not only Raava, but Vaatu as well.
I've seen this idea across the internet a few times, but it hasn't been thoroughly explained correctly. Some say that the Avatar is suppose to bring balance to the Four Nations; others say that the Avatar is suppose to bring "peace" to the world. The latter is correct, but, true peace can't be achieved without "chaos" (Vaatu)/evil. It's pretty much the reason why these worldly conflicts keep arising; things are not balanced, and unbalance causes problems, regardless of how much order and "peace" (Raava) try to stop it.
I see this as being a possible revelation in a possible sequel to Legend of Korra. Maybe, the next Avatar to succeed Korra, will be more open-minded and understanding of true order (balance). With Raava being the sole inhabitant of the Avatar, "peace" is pretty much what she wants it to be, as well as "chaos" being Vaatu's own view of peace, in a sense.
In a nutshell: Raava and Vaatu have to come together to form a being of "balance" – not "peace", "order", or "chaos".
And finally: this is just my view of the Avatar and what it should be; it may be right or wrong, so... yeah.