I think it's a political move more than a technical one. WinRT lets MS say to Intel that Intel has to care about the tablet market and low-power processors or otherwise ARM will get a foothold in Windows (giving them easy access to the low-end of the laptop market in the next few years).
Of course, this would have worked a lot better if people cared about WinRT at all, but I'm sure that was the idea.
Margins on CPUs for tablets and phones are not all that high. Intel enjoys pretty amazing margins on their desktop market, so they aren't missing too much there. They just need to keep ARM away from their bread and butter of profits, which probably can be explained by their current push to keep ARM out.
No, but tablets and smartphones are starting to affect the PC market. Desktops are dying, and laptops are stagnating.
For a lot of people, a smartphone or tablet is sufficient to meet their computing needs. Especially with ability to connect a TV and keyboard, and a number of small ARM based "mini PCs" starting to arrive.
Of course, this would have worked a lot better if people cared about WinRT at all, but I'm sure that was the idea.