Is X11 still used today?
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).