Well first of all I don't think it's just old school games. I mean, the majority of sports games would work with the sideways controller setup, right? It's really only games that require a player controlled camera which can't be played that way which, to be fair, isn't exactly the majority of new games today. Certainly a big chunk of them, but there are plenty of games that don't require dual analog.
But second, I don't think the local multiplayer out of the box thing is the primary selling point of the detachable controllers. The detachable controllers make the most sense to me as a way to simulate touch screen input when the screen portion is docked, so that there aren't going to be any games exclusive to either mode. The selling point therefore will be: "here are your controllers, go play your way."
It's a simple message and it honestly sounds like a very comfortable and functional control scheme (dual new wiimote+) assuming they get the ergonomics right. Split controllers are the future man.