Hrm, sakura is important for one main reason, she is one of the focal points of the rivalry between sasuke and naruto. Every story needs a character that provides motivation to the protagonists, and Sakura is the personification of the rivalry between naruto and sasuke in many ways.


Another thing to keep in mind, is that Naruto probably has more chakra potential then any genin we've met. A good analogy (wierd, but go with me), is the guy who is a champion hot dog eater. Naruto overflows himself with chakra from the nine tails enough times, eventually it's going to stretch out the container that normally contains a certain amount of chakra, and Naruto will have a greater chakra capactiy. One key point in the series for me was when he did the mass replication against Gaara. From my perspective (also the way they colored the anime), he did not use any nine tails chakra to do this. That was an enormous amount of chakra to do that.

The same thing with the rasengan. One of the comments made by Tsuanade is that naruto's attmepted uses of the rasengan were never containing the power of the fourth (while he was training). Saying that the rasengan doesn't take much chakra I think is a bit off kilter in compared to chidori, mainly due to how much chakra that Naruto was using to "charge" his balls of chakra during the training.