If they really do this, then anybody who has shared access to my library can play the games I own. To play those games, they must be downloaded and installed. I don't think Valve will create a CEG-based starter for every shared-accessed game, because that would be a huge amount of work. So in theory the borrower can start the games on our list and play them even when the lender is playing.
As I see it, that could be legally ... not so great. I don't want to do that but the possibility came to my mind. Any opinions?