I would like to add another (extending) use case:
I have a local Owncloud installation on my NAS, Raspi, Home-PC, that has access to all the terabytes of local disks laying around, maybe not always available. Then I have an Owncloud in the web (root-server, whatoever), always available, with a good connection (upload & download) but limited space, that serves all my phones/tables/PCs etc. with files/contacts/calender/etc. The home-Owncloud is hooked into the web-Owncloud via federation, to have everything accessible. In the Web-OC I can decide which data I want to have always available there (similar to the checkboxed-tree in the Windows-Client).
I don’t want to maintain to sources/client-folder in my sync-folder, one file, one URL, independent where it resides (this should be transparent to me as a user/application program). If the home-OC is not available obviously I cannot connect to the content, if I did not check the box to mirror it on the web-OC (which is included in a completely different User Story as well).
But if the home-OC is available, I would not like to load the file (e.g. a movie) from my disk through the home-OC to the web-OC (with limited upload bandwidth) and back again to my PC 3 meters away. The routing shoud find the local LAN shortcut.
(I guess this needs to be solved mainly as a proxy/routing issue. But I am not an expert in this. The Sabre guy who was on the OCC 2015 in Berlin might be knowledgeable enough with this topic to help.)