Uses for a second screen on the back :
1) Present an image of you being interested in the person you're with, while you're busy using your phone on a date
2) Put LCD/OLED on the main side, and e-Ink on the other. Use the first side for normal usage, and the low power e-Ink display for reading books etc.
3) Wrap the entire phone in Samsung's curved AMOLED displays. Change the entire color / background / animation of the phone body itself to fit your mood. Something like this early 2007 Nokia video at the 1:04 mark ...
Dual screen concepts pop up all the time.