It's not that you can't play the same game simultaneously - you can't use the same library simultaneously. If you're playing Borderlands 2, your friend can't be playing your copy of Skyrim.
Ah, you're right. Well that'd suck. I mean I guess I understand why, but it'd be a bit cumbersome. 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?