I'm assuming this is how Nintendo aims to win back hardcore gamers; by adding functionality to what many developers consider a limiting controller. On top of that, how do you convince casual gamers that MotionPlus is better or worth how ever much it costs? I'd bet that Nintendo's thinking is that they don't have to. Casual gamers aren't complaing about the Wii, everyone else is. Still, I'm having a hard time justifying this. If the nunchuck plugs into the MotionPlus module (which it better or it's useless), then Nintendo just introduced the world's first $80+ 1st party controller. How can anyone be happy about that?
The problem with MotionPlus is the same problem with any peripheral: it's extra and who wants to pay extra? Additionally, Nintendo needs a major title to vindicate the add-on's existence. People have said Zelda, but the next Zelda would have to wow audiences more than Ocarina of Time to make a difference. Good luck with that. Packing in MotionPlus with Zelda like they packed the Wii Wheel with Mario Kart would certainly help, but Nintendo would have to sell the game at a loss or pass off the difference onto the consumer, both of which has its pros and cons. Ultimately, launching MotionPlus is risky though I suppose being risky is what made Nintendo relevant again.