You have no base to prove that her powers are temporary. For all we knoe, Naruto and Sasuke's powers could be temporary, too. But all three of those possibilities are highly unlikely, so let's not dwell on that. And Sakura was doing what she thought was best throughout the entire battle. She is one of the best healers that Konoha has, so she was saving herself from the battle, enabling her to save other people as well. She is not useless, you're just upset that Sakura is finally catching up to Naruto and Sasuke, possibly overshawoding them a bit, in the process.
