You realize you don't need to use the defaults command, right? You can use a text editor or the property list editor that comes with the dev tools if you so wish.
https://en.wikipedia.org/wiki/Property_list
Oh and, mucking around in a plist still won't completely screw up a system like mucking around in the registry can.
Mucking around the registry won't screw up a system, same as the plists. Some plists can destroy important boot services, just as changing some parameters in the registry can.