To be honest, the windows 8 setup is probably the most compelling reason to buy one of their tablets (the Surface Pro though), the interface is perfect for tablets (or even TVs), and you get complete compatibility with any windows application.Everything about this unified platform things sounds like a bad idea to me. Just look at Windows 8 trying to make an OS for tablets and standard PCs. You get a middling OS. A handheld console hybrid would just be a middling game device in my opinion.
But you also get a full desktop if you need it.
I can see MS carving out a huge chunk of the tablet market because of windows 8, as these laptop/tablets converge (tablets with removable keyboards).
But your analogy doesn't really hold well to the whole nintendo portable/home console thing.
Both handheld and home consoles are game machines, and they display games on a screen with a control input. Whereas the windows 8 thing is more about metro versus desktop. Nintendo will never heed a mouse based desktop style UI to confuse the matter. It's a gaming platform, whether portable or at home.
A better analogy would be android, where it's primarily a handheld device, often a tablet, and now a days on your TV, via HDMI, or even wireless dongles, the platform is the same, so developement is very easy, but the size/distance of output is different, and some of these devices even have different skins to optimize the experience.
If Nintendo would do something like that, it could be even better as it would be from a single vender, with a single vision, rather than the multiple vender setup android finds its self in.