Wii is no substitute for 'real' activity
CAN playing games on a Nintendo Wii make you fit?
Answer from Dr Tracy Kolbe-Alexander (from Research Unit for Exercise Science and Sports Medicine at the University of Cape Town): The Wii Fit is better than the Wii Sport because the activities have a higher intensity. I also think that someone who chooses Wii Fit is less likely to try to cheat the system - in the Wii Sport you can get a high score without completing the whole movement, for example boxing by just moving your wrists.
But the bottom line is that you shouldn't expect benefit if you are going to try to do as little movement as possible. I would also warn against using this as a substitute for "real" activity.