Perhaps it's more to do with application compatibility. If they create 2 separate OSs with different UI systems then you require developers to create 2 differen

Perhaps it's more to do with application compatibility. If they create 2 separate OSs with different UI systems then you require developers to create 2 different versions of their applications if they want to target tablets and desktops. A big selling point of Windows has always been "use the same OS at home and at work". If MS are guessing that home users will largely go for WinRT devices over full on Windows 8 boxes they lose that advantage if there is too much fragmentation as you will have completely seperate app ecosystems for each. It is also an OS for hybrid devices such as the Intel based Surface tablet.