The Start Screen already is a start menu....
No, its not.
The start button is a UI element of the desktop which can run commonly used desktop elements along with control shutdown of the computer.
The start screen is a UI designed to be used in place of the desktop to manage and control apps and applications through the use of tiles.
Things will start to make more sense when you understand they're(the mobile interface and the desktop interface) two distinctly different interfaces for controlling apps and applications. They simply vary in how they accomplish that task.
The Mobile Interface is analogous to a TV Guide. You pick the channel you want to watch, it loads the app via a tile in that channel at full screen, and you can flip between apps as if you were Channel Surfing. They're even putting more abilities to have multiple viewports as if you were a producer at a tv network. Which feed do you want in which tv?
The Desktop Interface uses resizeable windows to contain and run its applications, which are loaded via icons, and allows the user to dynamically control where they want the particular windows.
It really is simply two different paradigms and ways of controlling a computer. The mobile interface works better on mobile devices, and the desktop interface tends to be far superior on workstations. Either way, there is no reason they can't exist in parallel, so long as they do whats needed to prevent bleed over.
Apps should be windowed, and to be perfectly honest.. the Start Button should return. Instead of the Start on the Charms menu, have it be Desktop if in the Start Screen and Start Screen if in the desktop. So you can flip back and forth. If you hit the window key it toggles to whichever one you aren't using. But having to flip over to the Mobile Interface to launch a desktop program which then launches the desktop interface is a fairly retarded way of setting up your operating system.