Thank you for your replies.
I understand how Family Sharing presently works. However, given that Family Sharing is intended for adults as well as children:
(from: http://www.apple.com/icloud/family-sharing/)
it would be preferable, in my view, if adult members of a family group could enjoy billing autonomy, should they prefer it.
It is already possible for family members to hide purchases from other members of the family group.
That feature seems irrelevant if their purchases cannot also be billed independently. I can imagine various examples where this may be preferable (for example: if Mr or Mrs X wishes to buy a self help book from iBooks that they do not wish their spouse - the family organiser - to know about).