Why Sakura Haruno Does Not Work
Sakura Haruno, the third person on the main team of Naruto, is definitely also the third wheel. I would say she never managed to evolve as a character the way Naruto and Sasuke did, and now she is outside of the essential plot, staying on the sidelines. But why? This is why I believe Sakura does not reach up to the same levels as her companions.
The Central Theme of Naruto
Now, please bear with me for a moment, as I would like to say what I believe is the central theme of Naruto. Naruto is basically set in a post-war world, and the story is about the people colored by the dreadful wars of the past. Naruto and Kakashi both lost at least one parent to the wars, as did Orochimaru, Nagato, Sasori and a lot of others, and these deaths would set them on the paths to the ones they are today. For example, Orochimaru's obsession with immortality seems to have started with his parent's deaths. Naruto and Gaara were both turned into living weapons which were useless when the war ended. The massacre of Sasuke's family could be seen as a reaction to the wars, with Itachi having been traumatized in them.
We see the world that war has created, the problems and struggles that still remain, and the traumas, wishes and fears that the people influenced by these problems have. And then there is Sakura, who seems completely removed from all this. We do not know much of her family, but for all we know her parents are alive and well(she briefly talks to her mother at the very beginning of the series, but no family is mentioned after that). She has no relation to the war, nor with any struggles. Now, one might question why Sakura's parents have never been shown, and I would guess it is because Kishimoto could not think of a subplot to put them in. Still, this means that Sakura is largely removed from the manga's central theme, and that her past is partially empty. And that is a big handicap in a show with so many flashbacks, where a person's motivations and personality is largely shown to us through their past.
So what conflicts are there in Sakura's life? Her past as we know it centers around two characters: Ino and Sasuke. Now, Sasuke was just a boy she was in love without really knowing him, and as far as we know they had never spoken prior to the show's beginning. Still, he does symbolize love, which is one of her struggles. Sakura's fanlike adoration going over to full love, only to then have her heart broken as her beloved turns into a criminal is one of the most interesting aspects of the character, and the meeting between Sasuke and Sakura after the summit of the Kage's was very emotional. Yet this relationship suffers from one thing: Naruto.
Now, enough jokes about Naruto being in love with Sasuke has been made already, so I will not go there, but I will say that Naruto's relationship with Sasuke completely overshadows Sakura's. This makes sense, Naruto is the main character, but it means that Sakura does not evolve much. In the meeting between Sasuke and Sakura I mentioned, Naruto quickly arrives and when he does, the story starts focusing on him and Sasuke, not on Sakura. When Sasuke leaves Konoha, he and Sakura has a scene, but while Sakura is crying, we start focusing on Naruto, and Sakura never gets any more development. She stays behind, because this is Naruto's story.
The next conflict is with Ino. Ino was once a friend of Sakura who then became her rival, and the two of them fight and bicker constantly. Her conflict with Ino is overcoming her inferiority complex, brought on by Ino always protecting her. But this all ended more than a decade ago, during the Chuunin Exams. So this part of Sakura has mostly been finished.
The final conflict would be Sakura's weakness compared to the other characters. Sakura has never had the same strength as Naruto and Sasuke, a fact she is painfully aware of. Once again, this may partially come from Sakura's lack of a past. Most of the younger characters inherited jutsu from their families, but Sakura does not have anything like that, nor does she have a demonic trapped inside of her like certain others. Early in the manga she was the smart one, with lots of theoretical knowledge about the ninja world, but even then that hardly translated to fighting power. But at least she recognized this and tried to change herself. Sadly, this did not work out well.
Sakura modeled herself after Tsunade, but unlike Naruto and Sasuke she did not have any particular skills herself, so she basically ended up as a slightly weaker version of Tsunade and the healer of the team. For her fight against Sasori this worked well, and she really kicked ass. Sadly, this would be the only time her medical knowledge would be essential in defeating an enemy. Then she is demoted to full time healer, a thankless job in fiction. While in real life and games healers and doctors are essential, in fiction the fighters will always heal as the plot demands it. Naruto is already said to have superhuman healing abilities.
So this last plot following Sakura ends with a sigh, as Sakura simply acknowledges that Sasuke and Naruto are stronger than her. And for now it seems that that is the end of Sakura, I do not foresee that she will suddenly become extremely relevant in the near future. In the end, Sakura was never a main character, and she was never given the attention from the author to be one. Sure she hung around with the main characters, but she did not have the depth, the past and the powers of the others, and in an action manga, not possessing any of those those qualities will make you useless. Quite sad, really.