Depends on the tasks to accomplish. If you're concentrating on a single task in a single application with a lot of information flow, a full screen window will enhance productivity (less scrolling and searching around). If you're concentrating on a or multiple tasks that require interaction between many different applications, that's where multiple smaller windows overlapping or in a mosaic will be superior (less tabbing around windows to get information).
And yet again, a 3rd scenario : A single task or multiple task that requires interacting with multiple applications that have a lot information flow. What solves this ? Something Microsoft never got : Virtual Desktops. Hey look at that, Apple's Spaces (of course, Apple was quite late to the game on that one, but they got there and better late than never).
That's the beauty of choice and I don't see how Apple implementing a better "full screen" mode in this day of Windowing systems is a negative. It's now up to developers to make the best use of it and for users to use it at the proper times depending on their workflow and habits.