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.
That seems to be the main perk, yeah.
It'd be nice if two people could concurrently play different games in the same library, but I wonder if that's part of Steam's licensing terms, in that part of why they can offer such deep discounts is that there's a guarantee to the publishers that you can only be playing one game in your entire library at a time, so if you had a library of 10+ games you couldn't have 10 people each playing a different one. I could see publishers and developers and such really balking at such a system.