It adds tons of value. 
1. We don't modify config-default.js. This means we don't modify the git tree behind users back. If we modify config-default.js on our side user might have to deal with merge conflicts or we might revert her changes.
2. We don't force the user to copy the original config and modify the file. We just ask the user to provide a list of values that wants to override.
3. the pay is negligable and this is usability and attention to the detail for the developer.