I need to install Photoshop so I can explain this a bit better...
Honestly, if that was how Windows 8 was done, I think it wouldn't had solved many things. The main problem that I see in mockup ideas like this is the simple window management. Think of this, what if you had about five concurrent open windows on the Desktop. You need to open an app from the metro Desktop, and open Photoshop. You'd need to hit Show Desktop on the Taskbar, open the app (which I'm assuming is in a window), then manually open back your open windows, then hit Start to open Photoshop. It isn't ideal of a scenario.
This is why I feel it's better how it was designed in Windows 8, I'll extrapolate.
Think of the Start Screen, concurrent open windows on the Desktop, and Desktop background as separate layers. First, you have the Start Screen that is on top of everything, windows and Desktop. You click on the Desktop tile from Start, you enter into the windowed environment layer. Hit the Show Desktop button on the Taskbar, you enter into the Desktop layer to open a folder to launch a shortcut or photo file you're working on. Now, you may or may not have things on the Desktop, depending on how well you use the Taskbar. Back before Windows 7 Taskbar, people used to use the Desktop background to hold icons and folders, basically like a start launcher. Now, it'd be tedious to hit Show Desktop all the time to minimize your open windows, and then to open a program, and have to manually open all the windows back up. This is what the Start Screen is, it takes the elder purpose of the Desktop background, slips that lower layer and puts it on top so you only have one thing to click on and open a program, go back to the windowed layer with ease.