With regards to why X11 is a useful thing, I'd say there are three things that are most of interest to me:
1) the ability to run ported UNIX software (with their native GUI) under OS X
2) the ability to run software remotely from a UNIX server (using its GUI) on my Mac
3) theoretically, the ability to let OS X run ported UNIX applications and "forward" the windows to other machines located elsewhere (that is, multiple users could see their own work on their own monitors, though the work is being done remotely by OS X)
Points 2) and 3) rely on the fact that the X Window system can be used to forward displays to other machines. That is, the window associated with an application being run on a UNIX machine is projected, as it were, on to a remote computer. As of yet, OS X and its GUI (Aqua/Quartz) cannot natively do this. Using things like OSXvnc, you can indeed display the image on your monitor remotely on someone else's machine, but this is not the same as each user having their own, unique display, acting independently of one another.
edX: phew, glad my blushes were spared as my experience could be of use to someone else!
I was perhaps too quick to associate the Fink incompatibility with this issue, as it never occurred to me Apple would go back to a relatively ugly window manager after making the first release of their X11 app rather more attractive! Anyway, using the installer was the quick and easy way of doing things. The "less friendly" way of would have been to go to:
/private/etc/X11/xinit
and then edited the xinitrc file so that "exec quartz-wm" is used instead. For instance (WARNING: only proceed if you are comfortable with the command line, as you will temporarily assume root privileges):
(1) sudo pico /etc/X11/xinit/xinitrc
(2) add a # symbol in fron of the line with "twm" in it (i.e. "twm &" becomes "#twm &")
(3) add the following line below: "exec quartz-wm"
Hope this is of help to someone.