What about a web browser? Some web pages are designed with a fixed width, but others, like the AT forums, are designed to use all the space available. If I'm browsing the web and not doing anything else, I want to put all my screen real estate to use for browsing, especially if I'm using a smaller screen like a laptop.
I think the real reason OSX does this is it's simply not as easy to switch between windows. In windows I can slam my mouse to the bottom of the screen and click on a nice wide button (which doesn't move side to side) to switch to a different window. On OSX, most people have the dock on autohide, so I have to move mouse to bottom, wait for dock to come up, find my app, and slide through the dock and click on it. UNLESS none of my windows are maximized, then I can just click on the window i want.