You'd think the idea behind sharing between family members would be so that several people in the same household (like say three or four kids) can play on different devices at the same time. If they had to take turns then it rather defeats the purpose, no?
I think perhaps the idea is more that the same household sharing one gaming PC (which is probably typical?) isn't mixing everybody's saved games and achievements together.
If that's the case, you'd think it'd be easier and more beneficial for the end user to implement a multi-user profile system within the same account. That way you wouldn't need to log out and back in to switch between accounts and you wouldn't be restricted with what you can and can't share.
No, I think this is about more than saves and achievements. I think it's them testing the viability of putting full games on the Steam market system.