You are quite right that Windows 8 is two entirely different UIs mashed together.
The idea is simple: Microsoft wants tablet computers. Existing tablets that use the regular he regular windows UI have been tried but nevr caugh on. Tablets that use touch-specialized UIs have caught on.
Microsoft wants to be different from Apple. So instead of having a Windows tablet be a scaled up Windows Phone OS device (Like an iPad is a scaled up iOS device) Microsoft decided to make it a scaled down version of their Desktop OS. To do so, they decided to basically replace the existing OS UI with Windows Phone inspired UI. Of course, they needed to have backwards compatibility, as well as compatibility with their other major apps (like Office and Visual Studio), so they kept the old UI around too, albeit somewhat gutted.
Here is what I predict will happen: Desktop application developers will almost completely ignore the new Metro-style app options, and only lightly metro-fy their apps to blend in with the redesigned Office and Visual Studio apps.
Tablet application developers will create metro-style apps, but only care about the user experience on tablet devices, so the applications will really suck on a non-touchscreen Desktop PC.
Casual-game developers will target metro-style apps for both both platforms since they can wdo so with minimal effort.
I'm really not sure what traditional games developers will do. They may target Metro on the PC, which will piss off many gamers who like the option to have windowed games, rather than only full screen. Or they might target the traditional desktop style for the PC. If they target Metro, they may provide settings for a touch based control scheme and to drop the graphical quality substantially in order to allow for more-or-less unmodified tablet build. If they target traditional Windows, they may either ignore tablets, or treat they as a second class porting target, with the usual gutting of the game when they perform the port.
In summary, my prediction is that except for Casual games there will be minimal overlap between Metro-style apps that work well on a tablet, and those that work well on a desktop, which is exactly the opposite of what Microsoft is counting on.