Interesting conversation. I've always kept my door shut but I think that is to keep light and sound out of the bedroom. I've given a lot of thought to locking the door and even bolstering the lock. The thought being that I would have more time and I would be more certain that a bad guy is on the other side of the door. Let's face it, friend and family are not going to kick down an internal door without announcing themselves in some manner. I pretty much thought once the door opens, action gets taken to stop the threat. Bad logic?
Also, for me, the kids are gone. I would not have the same thought if the kids were in the house and certainly would not be locking my door at night with kids in the house.