Whiteowl--I'm with you. When someone says "on your left" to me, it's a piece of information to alert me to their position. Sometimes it's because I was daydreaming and had drifted out. As a general rule, both when driving and cycling, I tend to stay to the right. It's not worth getting into an altercation with an angry driver and it's just plain courteous. And yes, I think we've worked ourselves around to the conclusion that it is all about common courtesy.