I can think of reasons why microsoft are trying it but I think those reasons are a bad idea.
1 - they get desktop users to use the new interface and it then sells more windows phones as the users then want the same interface on their phones.
2 - they are focusing on the mobile market as thats where the consumer growth is and are confident that their monopoly on the desktop market will allow them to release a substandard OS and still get adequate sales.
My own view is this.
If the time comes and windows 8 remains inefficient to use on my desktop and windows 7 is also EOL, for the first time since I have started using pcs I will be considering an alternate OS to windows, probably some form of unix. I have been using pc's since the ms-dos days so a very long time. Since microsoft would be putting me in a position where I have to learn a new GUI then obviously I would be much more willing to try alternate GUIs such as gnome.