Nintendo has always been a, "Throw (the product) out there and see what sticks," kind of company. From the Power Glove to the Super Scope, to the 64DD, Nintendo just manufactures its imagination (sometimes seemingly regardless of market demand). Why does the company release peripherals like R.O.B. and the Donkey Kong Bongos (admittedly fun, but with limited capability)?
The Wii Fit is a Power Pad
The Wii Fit is a very impressive piece of technology. It measures acceleration (via weight) to simulate on-screen movement. The device can determine your head movement by the shifting weight of your feet, which is amazing. The gaming potential is as limitless as it is for the Wiimote (particularly if used in tandem). Sadly, Wii Fit is still just a fancy Power Pad. The Power Pad failed due to lack of support from developers (and therefore a lack of titles), lack of interest from players, and ugliness. But hey, at least the Fit is not ugly.
Wii are not Fit
The Fit was dead before it was even announced. The Wii Fit contains the word, "Fit." People hate fit. In the United States, at least, we live in a culture where pills melt away fat and surgery can cure obesity. Forget dedication to exercise and commitment to dieting, the Doctor will take care of everything! Sure we would like to be fit, but not if it takes actual work. Nintendo's ace in the hole might be the Feng Shui and yoga crowd, but the size of that segment of Wii owners is unclear.
Alternative Products
It appears Wii developers are already adopting their own proprietary fitness hardware, creating competing hardware for the Fit before it even hits shelves. It might be crappy hardware and software being released by third-parties, but holiday shoppers are not known for doing their research, putting a potential dent in the target demographic of the Wii Fit.
Successful hardware peripherals need...
...to be intuitive. If the controller or input device the firm creates is cumbersome or confusing, the device will ultimately end up gathering dust. Sony had to dump their original PS3 controller design when they realized it was, well, garbage. Conversely, the N64 controller was easily one of the most intuitive interfaces ever designed for a console, anyone picking it up knew immediately how to hold it, and it was extremely versatile depending on whether the game required a digital or analog stick.
...distribution and marketing muscle. Sometimes a peripheral can get away with being trash if it has solid marketing. It does not necessarily make it a good peripheral; it just means that it will sell. Look no further than the Aura Interactor, one of the most craptacular peripherals ever. Effectively a giant purple vibrator you strapped to your chest while playing games, the company sold over three million units because it had an effective in-store marketing campaign and country-wide hype to push its products.
...to be fun. The R.O.B. was pretty cool growing up, but it did not really do much to make gaming more fun. If anything, you felt like more of a loser for needing a robot to be your friend. Conversely, the Game Genie was practically the perfect peripheral. The device modified your actual gaming experience by changing the way your games were played. Whether your character was able to fly for the first time, had infinite lives, or all the best equipment, the Game Genie was incredibly fun to mess around with after completing a title.
...software support. The ultimate killer: If there are no games there will be no sales of your peripheral. The NES Max, Game Genie, and even the craptastic Aura Interactor added to the gaming experience of any title in the gamer's library. Conversely, buying a Gamecube Microphone or SNES Super Scope would only work with a few games, limiting their target audience from console owners to console owners that liked to talk to their Pokemon or play rail shooters, respectively.
...to be included. The key to selling a peripheral is to include the hardware and bundle it into the cost of either the system or the software. Donkey Kong Jungle Beat would have been nigh impossible to move had it not included the controllers, as if it were not a tough sell already. Even the fabled light gun, the NES Zapper might not have seen use without being bundled into the original NES system. Packaging peripherals with their respective software makes the cost of purchase seem like a better deal to buyers, even when building in the price.
I hope the Wii Fit surprises me. I hope Nintendo puts together a strong lineup of first-party software to support the Wii fit. I would not mind dodging bullets and sword strikes while hacking away at my enemies with the remote. Thinking of the Jedi possibilities makes me drool. Unfortunately, for the aforementioned reasons, I do not see the Wii Fit succeeding.