I think he is just interested in the quick powerup for his Curse Seal. It going from Level 1 to Level 2. Maybe he also wants to learn some of those forbidden techniques from Orochimaru. I don't think he plans on staying with him long term, kind of use and abuse him.

I still think they hav built Itachi's powers up way to high. It is almost like he is unreachable for Sasuke now.