I apologize if this was already discussed. All this time, maybe because I never paid enough atention, I thought obito wanted sasuke just as another pawn to be used for battle. I did not really cared, also, and thus never took the time to verify if that was the truth.
But a while ago, after re-reading the manga, I came across the part in which obito talks to zetsu after dispatching sasuke to the kage summit. There, he tells zetsu that he has no need for sasuke growing stronger than pain if he's not easily tamed. (and zetsu interpretated this as sasuke being a contingency plan for obito, all this in the rinne tensei context)
All this made me realize that pherhaps obito wanted sasuke as a new nagato. So, in the case the later failed, and sasuke proved to be tamed, obito could give him the rinnegan and have him use the rinne tensei. (he later says that he's not planning on sync-ing him with the gedou masou for a while, so he probably planned on doing it after finishing the war, which we all know took a turn even obito did not expect).