Many power-users of OS X put the dock on the left or the right—not on the bottom. The vertical space is precious. The horizontal space is superfluous for many/most apps.
Right now, my browser window is vertically maximized but doesn’t go too far to the left or the right. My 15” screen has a native resolution of 1440×960—1440 pixels is much bigger than most web pages. (The 13” Macbook has a native resolution of 1280×800, which is a little closer to normal web page size, but still a bit wider than the average site.)
When I use Windows at school or work, the screen size is usually smaller (1024 px horizontally), so maximizing makes much more sense.
Some apps, like Photoshop, have floating tool bars. I don’t want my image to be behind those most of the time. But Photoshop has a real full-screen mode on the Mac, right? (I haven’t used Photoshop in a while, so… I could be wrong.) When I’m doing word processing, I usually have the window maximized vertically, but not horizontally. There isn’t much that I do which requires the full horizontal screen, and in those cases, specific apps (WriteRoom, Quicktime, DVD Player) provide full screen solutions.