I actually think Airbus also recommend having the lights turned on during the cruise at night, this was in one of their getting to grips guides on fatigue.
Personally for me unless there is weather, or I am looking at the stars or something... then I have them turned on.
As others have pointed out sitting in the dark is not much use at all, as your eyes are not focused much beyond the windshield so wont see anything anyway.
Fatigue is the greater risk.