Night mode is actually not a gimmick. It works best when listening at low volumes. What it does is make softer sounds louder and louder sounds softer, bringing the overall sound to a more constant level.
This can be handy when you're watching a movie late at night and don't want explosions to wake your neighbors/housemates/kids etc. It should also make it so that you can hear whispered dialogue without having to boost the volume for those passages.
I personally never use the feature (my Onkyo has it), but it may be a life-saver after the baby arrives.
