I think that Kishi added the whole symbolism only so that he can give some importance to Sakura. Let me make it clear, I don't say that she doesn't have any but to be realist, she had no place in that whole symblism. The importance was to show the whole Yin-Yang concept, what's Sakura's role in all that? NONE! She jush happened to be there and Kishi had to incorporate her somehow since it would be funny to show the Yin- Yang concept with a third, random person, being ιν the equation without being incorporated in it.
EDIT:
After reading the answer given about being the bridge between these 2, I accept it as a better sceanio and afterall better the way it was done.