I knew that Zelda Four Swords had a single-player mode, but I thought it still required the link cable. Not that I would have bought the game if I knew that, since I likely wouldn't have ever had the chance for multiplayer, though if I ever see it cheap I'd get it just to have it. I'm still hoping to find the mythical link cable at a GameStop, as well...
As for using the 3DS as a controller, I think it would be neat as optional functionality, but it shouldn't be a requirement for any multiplayer games. I don't think it will be, since it was said that Wiimotes and Nunchuks can be used for multiplayer, but I still hope to see multiple screen controllers supported.