Why there should be only one window manager for windows and os x is beyond me too. X has window managers because it is not the job of the X server to move windows around. In and of itself, X already has the concept of windows. The only thing the window manager does is make the windows pretty and allows you to move them around. The number of window managers allow you to use what you like the most and what fits your computer.Why must there be so many window managers for Linux is beyond me.
Wrong. An operating system is what it says it is: It operates the computer, nothing else.An operating system, by definition is something that interfaces with the user through a GUI.
There is a standard. However, some window managers are non-compliant. Most of the common window managers are compliant and standardized. Some less known window managers might not be perfect but that is what you have to expect from little known programs on any operating system. The major wms such as metacity,kwin,XFCE's window manager, enlightenment, compiz, beryl, and compiz-fusion are all pretty much standard and there is not much difference between them. However, there are those that try to be different such as Xmonad, but you are not required to use that.You'd think that since most GUIs these days use similar window-based interfaces, there would be a standard for those.
You don't need a window manager to get squares on the screen. Try killing your window manager. The X server will still show the squares on the screen. However, they no longer have a title bar and so you can't move them. This is all a window manager does. It simply paints the title bar and allows you to move it.If the model most commonly used is that of a window, as a simple square on the screen, why are there so many different managers out there?
How customizable can a window manager be? In most operating systems, you can't even change the theme for goodness sakes. Contrast this with X, where the window manager can take on a completely different look. Also, have you seen the options that compiz-fusion give you? If that isn't customizability, then I don't know what is.The problem with all of them is that each offers a minimum amount of customization options to the user, but to be fully customizable, you'd have to install all of the existing window managers on top of a kernel and then switch between them.
I don't think you fully understand what exactly a window manager does in X. I would really suggest reading the wikipedia articles on it as they are really revealing as to the inner nature of window managers.