She's completely useless, she can't even destroy one eye and she's shaking like a leaf like a child. And Obito definitely didn't just ask her, he's saying hurry up to her, like he's been sitting there forever trying to hold zetsu back because she's useless. God how hard is it to use a kunai and move your hand two inches to stab the damn rinnegan. It's because she's never actually hurt a real human before, so she's being a child and doesn't want to do it. Think about it, sasori was a puppet, not a real person, and who else has she EVER fought and had to stab? No one, when pain attacked she killed a centipede or caterpillar, whatever it was, never a human. What a useless character.