The start screen and missing start button are indeed steps forward when implemented properly and I think Microsoft is most of the way there. I would be very displeased if they added the start menu or button back.
There is still a start button, and they made it larger. The four corners of the screen are the fastest and easiest targets to acquire aside from the pixel directly under the current mouse position. The lower left corner is infinitely larger and easier of a target to hit than the old start button. The only place this breaks down is inside a virtual machine (you can still use the corner to open the menu but it's significantly slower).
As for the start screen replacing the start menu on the desktop, I think it's still a net win. How often do you need to see your current desktop while navigating the start menu? I can't think of one occurrence in my ten or so years of using Windows on a daily basis. If the rest of the screen is not actively used for the task, why fill it with noise?
The start screen on all of the Windows 8 installs that I've used opens just as fast as the previous start menu. I primarily use search (Windows Key + Start Typing...) and it works better than the previous version by taking advantage of the additional space on the screen.
I'm not sure if this is even a story. What's a rational well-reasoned argument for bringing back the start menu or button other than "it's what I'm used to"?