Hey, when you use somebody, you better trust him. If you don't trust him, you better off not using him at all. Plus I think DDBen was right, the book was show the evident that Sai's exist. I don't think Sai willingly to trash it when it is not necessary (especially alone in front of the injury Naruto). The picture of his brother that he has long forgot has been drawn show that Sai is now reconnect with his emotion. Plus, with Kabuto's aid, it is true that Sai and Kabuto combine power may not be a match for Yamato, Naruto, and Sakura's combine power, but if Sai really want to, escaping with Kabuto back to the lair is not impossible. In there, Orochimaru could easily assisst them. Even without Orochimaru's assisst, Kabuto's knowledge of the lair lay out could easily help them to escape. So why didn't Sai do it? Why did Sai help catch Kabuto if he just fake out? Think about it.