Its not functionality exactly, its ease-of-use. Using the Start screen is inferior to the start menu in two ways -
firstly its not sorted and nested and cascading, its 'flat' (especially the 'all apps' screen, which seems ridiculous, I dread to think how many pages all my unplayed steam games would take up!).
and secondly it means continually jumping back-and-forth between two very different style full-screens, which just seems it would be very disruptive. I mean, say you have a browser, visual stuido and solitaire running at once on the desktop and you need to quickly use the calculator to work out something in relation to one of those. Instead of being able to start up calculator while not leaving your desktop and still keeping everything in view, you'd have to jump to that garish start screen and back again.
It just seems a silly way of doing things. And it seems as if the only reason it works like that is because MS want to force desktop users to become familiar with their mobile UI.
Its like a form of intrusive advertising, having 'BUY MISCROSOFT MOBILE PRODUCTS, THEY ARE REALLY COOL!' flash up on the screen every now and then.