why would you want multy display out? mirroring? because thats all it would be able to as it would not be able to render 2 different streams
Watching TV or Netflix and playing a game at the same time
Bring up youtube and watch a walkthrough while you're playing if you get stuck
Letting people play two less intensive PSN titles at the same time
Co-op on dual monitors instead of split screen (even if they have to be rendered at 600p each)
Sports games where you can browse a full playbook without worrying about your friend guessing the play you might have called
Card games where you don't want anyone to see your hand
Managing inventory etc. in multiplayer games like Skylanders or other Gauntlet style crawlers without having to interrupt play for everybody
Following the auction house while playing D3
Clever games/gameplay scenarios that haven't been explored yet because dual displays on consoles have never even been an option for devs (outside of the DS and maybe Wii U)
Pretty much anything that can be done with dual displays that isn't limited to a completely narrow "if I can't play Killzone @ 2160p I have no use for more than one display" thought process