Ran Eilam wrote to me: Under the category of "stuff you can generate from grammars", http://kaptain.sourceforge.net/ make a GUI dialog from a grammar that describes a command-line.
I guess it may be useful for really complex command lines.
From the description on the tool's site itself:
"Kaptain is a universal graphical front-end for command line programs. Someone writes a simple script (so called grammar) which describes the possible arguments for a command line program and Kaptain brings up a friendly dialog to the user to set up the command line."