I believe that Orochimaru will takeover Kabuto's body completely and had thoroughly planned it out. Orochimaru almost invitingly let Sasuke be in position to kill him and knew that Sasuke had met up with his older brother at some point. Call it a large irrational gamble....but Orochimaru is a meticulous planner and was always doing some kind of genetic science based loophole to become stronger. It would be epic to see a return of some sort....
Also.....if Orochimaru had fought Sasuke full power with no impending death from needing a vessel ever so often and two sealed arms (courtesy of Sarutobi).....HE WOULD HAVE MOPPED THE FLOOR WITH HIM!!!