That's the problem with Nintendo peripheral pack in games to date: they are not aimed at core gamers. And they really shouldn't be.
In order to make this add on relevant, it absolutely must have a compelling game of some kind released in parallel, and more likely as a bundle. This game must either require the use of Motion-Plus, or offer controls that are so enhanced over the standard remote, that it feels like a necessary enhancement, rather than just a control option.
The idea that it will be compatible with former Wii soft releases or even current or future soft releases is highly unlikely without the additional coding necessary to utilize the added functions of 1:1 control schemes.
But this is not something that can just be "tacked on" with even a soft update since games that are designed for 1:1 controls must be done as such from the very initial stages of design when it comes to everything from creating character animations (no way of getting around this without cheating) to user input translation. Otherwise it wouldn't really be 1:1.
Even without hearing the specifics of how this little add on provides 1:1 movement/control (I doubt it is just an additional accelerometer, which would just provide an additional swinging axis to the standard remote), I find the concept extremely interesting, but with reservations.
An add on peripheral is only as good or useful as the softs that specifically require its use.
Without that killer app, it will be about as useful as the Wii Zapper, which I gave its due before I stopped using it entirely because it wasn't necessary as a peripheral for shooting games. Completely optional. Due to poor design (access to two buttons, B trigger and an analog stick without changing grips) and the basic fact that it really didn't enhance the remote beyond making it marginally more stable to shoot with, meant that Link's Crossbow Training was the real reason to buy the peripheral. Which, it wasn't.
So while Motion-Plus sounds great and was more or less what I originally believed the Wii Remote was going to provide back when the Wii debuted, I can only base my opinions on the games that require its use (specifically made for 1:1 controls) or those that are GREATLY enhanced by its addition.







