Is X11 still used today? I have had X11 installed on my Mac for quite some time (as long as I have owned it I think) and have never used it. Do applications still use X11 or is out outdated at this point? Should I "learn" how to use it because I may use it in the future? The reason I ask is that I read that you can run X over SSH and I thought it might be useful to run a window manager to get some graphics from a remote machine. Then I realized I have never used X11 and launched it only to become confused. So should I use X11 and why?
The following is a list of popular programs that require X11:


Gimp - open source photo editing
Inkscape - open source vector drawing program
FreeCiv - open source version of the Civilization games
Scribus - open source desktop publishing software


Basically, if you plan on using only commercial software, you don't need X11. If you plan on using open source software, you may want to keep X11 around just in case. Many open source projects used to require X11 but now have native Mac versions (OpenOffice, for example).