Just to have a try: imagine to install proofread extension and to add Index: and Page: namespaces into Commons, and to allow users to use Commons as an optional proofreading lab.
No more painful alignement of Authors' metadata: there is Creator nmespace/template.
No more painful setting & aligning of book metadata: there is Book template.
No more painful localization: there is a powerful set of localization templates/scripts.
No more need for trick for multi-lengual books: they would be as simple as mono-lengual ones.
Obviously such Commons books need to be fully shared - as html rendering - into any wikisource project, just adding a message "Work can be edited in Commons", jus as it happens for shared media.
Alex