+1. Bicycling and motorcycling, my thought has always been if I have time to reach for a horn, I had time to reach for the brake, and I should have been planning on that person to pull out/slam on brakes/swerve into me/whatever.

Plus if you just honk, you're expecting the other person to realize who is honking, and then take a proper evasive action after orienting to the noise. That's a lot to expect from another person, especially when you're putting your safety in their hands.