The fullscreen Start Screen completely and utterly destroys any workflow I may have, it's like getting a fullscreen ad thrown at you during a game or a friend pressing the "start" button during a COOP game. I'll be navigating a one line vertical bar on the left side of my screen still capable of watching any media I might be running simultaneously or keep track of my game should I be standing in a unsafe area. I also only have to look at the left side portion of my screen rather than looking around all over my screen, the standard Start Menu keeps things focused, the fullscreen Metro UI Screen is all over the place (I know you can organize things, but the argument still applies since you still will end up having programs on different parts of your screen, some you even have to horizontally scroll to the next page to reach.
The Start Menu keeps things quick and efficient (if you're digging down in the folders menu then it's faster to search for it, which means the reason you're digging is because you don't remember the name of the program in which case it's probably new as well and highlighted which is easy to find when you have dozens of folders in a small space). The Start Screen is all over the place and inefficient and breaks any workflow or immersion/focus you may have.