Well I guess I'd have to say I'm in camp #1. First and foremost control needs to be maintained so a collision can be avoided. When ever I am in traffic with my thumb covers the horn button too. Many a time vehicles start to move over in my lane because they're either day dreaming or never realized, or just totally forgot that I was in the next lane. When they do, I hit the horn and believe me, they can hear it because they swerve back into their lane like they almost hit a mack truck. I really believe the horn blast makes them more likely to look and make sure the lane is empty before they do it again. I've even had people waive and "mouth" sorry. If they don't see you let them know your there with the horn.
My wife, on the other hand, would prefer that I hit the brakes and slow down and just let them come over. When there is plenty of room for me to maneuver I even let them get a little closer, then hit the horn to give them a good scare. When that happens my wife tends to pucker up and get a little upset. But like I said, I only do this when there is no danger to me or my wife.
Just my 2 cents.