It's just that Sakura doesn't have a role here to do. She showed some of her power then she said that she wanted to heal the alliance, which to show that her healing powers are great also, meanwhile Naruto and Sasuke gone to do the combo attack. Afterwards, this talk with Sai... And it doesn't stop here which I'm sure that you know that.
The past two or three chapters were about Obito and Kakashi, Obito getting killed from Madara and the Kages trying to stop Madara till Obito reveals that he's the Ten Tail Jinchuriki. Yes, Sakura could go and stop the Juubi with Naruto and Sasuke but they were the closest, plus, it wasn't that big of a moment. The cause of Hashirama telling Naruto and Sasuke that they were the closest and go to stop Madara was to have Sasuke reach Obito and have this small action with Obito and to see Minato.
That's precisely the issue though. Sakura is intended to represent the characteristics of a heroine yet ever since chapter 634 her involvement in the conflict has been downgraded to supplemental duties as her teammates continue to act as the head of the Alliance's offensive force.
You can bring into discussion the myriad of qualities that a hero comprises of but what Sakura is doing at the moment, vacantly gazing into the masses of Jubi replicas whilst remaining completely inanimate, is not what would be commonly associated with a hero. Hell, the fact that there are numerous groups of inferior shinobi fighting in her stead isn't practical even from a medic's perspective. Would it not be more logical to become involved in the fighting if your aptitude for incapacitating large numbers of the enemy at once minimized the need for healing these weaker soldiers? Nevermind the fact that Katsuyu can restore any injuries on her own.
The main characters should get involved in everything, but not in every single action. Every main character has its role and a specific time for them to have it.
Yes, it's only because Sakura's status as a protagonist is inferior comparative to her teammates' that she has been put to the side for the time being. Despite the fact that throughout almost all of P2 following the Sasuke and Sai arc (her effectiveness in that mission is debatable as well)her role has been minor. The situation calls for her relegation to the background once again because Kishi is incapable of providing her with an active role for more than one just scene to help distinguish her from the likes of Ino, Kiba, or the other side-characters.
Even I won't contest to the notion that Sakura hasn't been truly acting like a heroine since the Sasori fight.
Sakura won't stop just after healing the alliance.
That is exactly what Sakura is doing right now. She has literally not taken a single step or carried out any actions at all since restoring the fodder to full strength. Having a brief conversation with Sai relating to the nature of Sasuke's actions doesn't constitute as effective or heroic behaviour.
She'll have other things and roles to do in this battle like Naruto and Sasuke having their roles.
No offense, but your assurances hold little value if Kishi doesn't offer any indication that there is more intended for Sakura aside from medical duties. Sakura is not acting like a protagonist at the moment. In fact, she isn't doing anything at all. There is nothing to suggest that her role will be expanded upon anytime soon. Especially with Naruto, Sasuke, Minato, and the Hokage clones being available to fight against Tobi; all of whom are much more powerful than Sakura.