Here is my take on this:
In today’s world there are many personal computing devices. The good old PC and notebooks, netbooks, tablets, smart phones. Though at a high level they seem to solve more or less same problem but each one has different use cases for their use. All of them complement together but one cannot simply replace the other. May be there could be convergence of these devices and can some of these devices obsolete in future. That is all together a different case for argument.
As said earlier each device is used for different purpose and for different scenarios. In that case their user interface should such that closely satisfies the target objective and very intuitive for that scenario. So in this context can we build common unified user interface for all the devices? It improves productivity and reduces the learning curve as the user interface seems to be same on all the devices.
But I would vote against of this as this will hurt innovation. This will really make us stop thinking the customization really required for individual device for what it has been built. May be this is the fundamental reason why Apple departed from its desktop OS and built a complete new OS for mobile devices such as smart phone and mobile phone and people embraced that.
On the contrary Microsoft has taken a unified approach for building a common interface for all its devices. If one has to think from Microsoft’s view point this strategy may be a right fit, as they have huge legacy of desk top OS and well established community. They definitely want the PC customers migrate to upcoming Windows8. May be that is the pull for Windows 8 on personal systems to have the same interface as like as mobile devices. This will also help them to build a single strong product and make it work on any devices. This is not anything new that Microsoft is trying. Earlier they have tried the same. They built the windows desktop interface on Windows CE and it was not well received by the users.
It is not simply, building the OS that matters, the OS future is dependent on the active community it can create. As Microsoft is late entrant into mobile OS though it has built earlier but failed to make a mark, it wants to capitalize on the existing user community to build applications. That may be a justification to be made but I personal think it will be good to have multiple user interface strategy for different categories of devices as each device has their own consumer strategy.