It's not. It's damned stupid as hell.
Boyfriend and girlfriend are in bed. They know something (ghost or demon) is haunting their house. Girlfriend vanishes from their bed. Boyfriend wanders the house looking for her, not turning a single light on as he does.
They're in bed. Something HOWLS somewhere in the house. They go to check on it, not turning a single light on as they do.
In bed. Something SLAMS their bedroom door and CRASHES on the other side of the door. They get up, open the door to check, and don't turn a single light on as they do.
Supposedly this is based on a true story. If so, I hope these two people die in the end, because anyone too stupid to turn on a light just... arg.
I expect I'll be watching many movies over the next week, I hope they'll be better than this one! (Invictus was darned good, by the way.)