Well ... it all depends on how fit you're currently and how fit you want to be.
I workout every day and am in excellent shape; I have focused mostly on cardio for many years with weight-training being (mostly) an afterthought. About 6 weeks ago (when there were a lot of news stories on new-years resolutions) I saw the same fact that I had seen a half dozen times before and decided to look into it; the fact was "Muscle is an active tissue and burns a lot of calories at rest".
It turns out that (to my surprise after a decade of training) that 1 pound of muscle burns 60 calories per day at rest ...
For someone who is not fit at all (in particular people who are inactive and obese) the minor ammount of physical activity that you do while playing Wii Fit or Wii Sports could (potentially) allow you to build 1 to 3 pounds of muscle. If you combine the 200 calories you burn from playing one of these games for an hour, with the 60 to 180 calories from added muscle, and have a diet which supports your current body weight while inactive you could lose a lot of weight in a year.
