No, because then there wouldn't be such a nice API for managing live settings from within the programs themselves:
http://developer.apple.com/DOCUMENTATION/Cocoa/Reference/Fou...
http://developer.apple.com/DOCUMENTATION/Cocoa/Reference/Fou...
Oh... It wouldn't
Additionally, Apple switched to a binary file format to improve performance in 10.5, which may be converted to/from the text format, or edited directly using the property list editor.
There's also nothing stopping a software author from documenting their settings file keys, and some do. However, most of the settings options you do see listed on that site are undocumented because they are unsupported.