I've recently played through all story battles in Ultimate Ninja Storm 2 a second time in order to unlock titles number 67 and 76 (as per this FAQ, see "Which name Shall I Use?").
While doing so, I've managed to get an S rating in all battles except for the very last one, where you fight an awakened Killer Bee. Since the AI is pretty brutal by this point and you are at a severe disadvantage, I haven't managed to get more than a B.
So is there any point to getting S on all story battles other than feeling good about yourself?