Fun but not essential
The Wii Wheel is a brilliant addition to the Mario Kart Wii package and adds a new dimension to the gameplay. Also, it is highly ergonomic and feels great in your hands. The 'B' button fits into the back to allow you to drift easily.
On the other hand, all it really does is provide a cover for the Wii Remote. You can easily play the game by tilting the Wii Remote as you would the wheel, so it's probably not worth shelling out on another in addition to the one you get with MK Wii.