When viewing a single document the View Mode control in the toolbar should disable the corkboard and outline view options if the document you are viewing doesn’t have any sub-documents. This is a better user-interface design than just bringing up blank corkboard or outline. I could be wrong if these are useful places to add the sub-documents or do anything else useful.
Many users would select an empty folder or a en empty document(to use as a document group) and use the Outliner or Corkboard to sketch all the sub documents they need. This is useful.
Tiho_d, thanks for clarifying.