It's less silly than you think.

You have to move around a fair bit, so you raise your heartrate, and work up a sweat, so energy is being expected. It's a low impact aerobic workout! And I'm working on improving my feet-eye coordination!

Plus, the game has a calorie counter (to be taken with a huge grain of salt, I expect)

Also expect to be a sucker for the Wii Fit board. (just a couple of strain gauges with a bluetooth chip, but still...)