1-A beat-em up where as you killed enemies they were knocked off the top screen into the bottom which contained spikes, moving blades, etc.
2-You could design or pick plays on the fly in sports game plus replays, stats, etc. could be displayed.
3-RPG'ers would LOVE the realtime always there screen for all the menus that rpg's require.
4-A game that requires you to backtrack such as a mario game could always shift the previous level to the bottom and allow you to have seamless access back n forth between the two.