We were discussing this in IRC, you should join the channel and take part in discussions
Yes, it is usually desired. The default value is not required, and it is not required in order for you to determine if the setting is there:
${SettingXML[filename].Set[Set].GetString[Test](exists)} - TRUE if the setting exists, NULL if it does not
${SettingXML[filename].Set[Set].GetString[Test,"Some Default Value"]}
Sets the default value in the XML file if the setting does not exist, and gives the value.
I can make some changes if you can present an argument for why it is better to have the default NOT stored.