Oh, and this reminds me: I was having a conversation with a local GameStop manager (I know... I had some time to kill), and we were talking about the second screen support on the PS4 and XOne.
I theorized that second screen support would be most useful and popular when:
1. Oriented towards a secondary player who was a casual or non-gamer.
2. Had a different experience than the primary player that would directly and immediately benefit them
3. Immediate drop-in and drop-out so that the secondary player was not chained to the primary player
4. Not necessary to core gameplay, yet valuable enough for the primary player to promote it
5. Needs to actually be fun for the second player (ie, a good game in and of itself)
Funny as it is, Puzzle Pirates is the model I'm thinking of. You've got these little mini-games, and it's basically drop-in, do some stuff for ten minutes, and then you're free to either play more or not. But to make this work right, you'd really have to have it in mind from the design phase, and not just shoe-horn it in.