>There is nothing preventing the desktop software from using a system-wide notification system.

But there is already one integrated into Windows 8, there is also sharing and searching integrated

Sharing

http://i.minus.com/ixWDWz6QqMRvG.jpg

Searching

http://i.minus.com/ieIlX6e9QyL16.jpg

shifting search to a different app/service with a click

http://i.minus.com/i8CxZ5i2wILEW.jpg

I'm not trying to become the Windows 8 advocate here but just showing how it has utility and value in many uses for me.