My idea was to create a site for every department inside an organization and use this type of sharing for collaboration between departments.
For example, I want to have a folder containing a "project" inside a site with default site permissions. That project would be a collaboration between users from one department and only certain users from another department. Outside users should have view and edit permissions for only that one project (folder) from that site and nothing else. When the project ends, the folder should remain in that site and only be visible within the site (permissions for outside users should be revoked).
Sharing problem could be solved by creating a new site for every project and adding users and groups as members, but then the system would be quickly filled with "project sites", besides, the projects should be inside and connected to their "parent sites". The same thing applies for sharing single documents.
I guess I could create one site with folders for each department but since permissions outside sites worked in Share, I wanted to know why shouldn't I implement it this way. By the way, I would probably make some custom browsing for "shared" files and folders.