Ok, I know that this topic may have been discussed to death, but I would like to throw out some opinions real quick.
I've recently noticed more and more of Sai and his involvement as a part of team 7. I've noticed some of the things that Sasuke has said, and kinda feel like everything is starting to make sense. We all are wandering whether Sasuke will truly be redeemed in the end or not, though it seems like he already has been. I'm sure everyone has noticed the little parts where Sai and Sakura show that they don't trust Sasuke, or where Naruto told Sasuke to stop burning the Juubi so he could save the bijuu, but was disregarded. Its obvious Sasuke is still very much so screwed up in the head, I dare say more than ever. If anyone actually believes he is normal now, or a good guy, don't fool yourself.
The only reason Sasuke saved Naruto from Obito was because he wants to kill Naruto himself. He said it " I will delete the past". Sasuke at this point is more than likely going to end up betraying Naruto after the Obito and Madara fight.
Back to my topic's point: I believe that Sai's character was created as a replacement for Sasuke. To Naruto and Sakura, Sai has been the true friend, his whole character was developed because of the love he has felt for his comrades. A huge part of the story was to redeem Sasuke and bring him back into the fold, however I don't believe he can be redeemed. I believe Sasuke could be if he stopped, but its just not in his nature. I think he will die in his fight with Naruto at the end of the manga. Sai was made to fill the gap. He is the true friend that would never betray Naruto and Sakura. The friend that would sacrifice everything for them. He is also the person who understands them more than almost anyone else in the manga.
In the end things may turn out different, but as of now, I really think this is how its suppose to be. Not to mention, Sai is the of the best things that has happened to the NS fandom.