I don't think the tablet could be used for any essential part of game-play because the Wii U only supports up to 2 tablets, and at this point it's expected to have at least 4 player local play. In a competitive game like Mario Kart it would not be very fair to give one or two players any inherent advantage/disadvantage in having the tablet as opposed to another controller.
I suppose Nintendo could add a second game mode that utilizes the tablet for single play or online play only, but this seems like it would add a little too much complexity over the standard upgrading graphics, adding characters and karts, and tracks. Really there's not much more that could be improved on a racing game. The touch screen track builder could be a fun supplement though.
