I have always said WiiFit would be a fad - a fad that lives about 18 - 36 months (like most fads) and sells insane amounts while it's all the rage.
There's absolutely nothing wrong with that. What it does is provide a large user base with balance boards for innovative game play ideas - at least hopefully innovative... We'll see what comes after the skiing/skateboarding... Tigerwoods might be an interesting use for it....
These results don't surprise me. Typical of any excerise equipment or heck, any game for that matter. How many people actually play a game 100% through? Not many... (unless it's a short singleplayer game like CoD4)