The point of an air horn is to attract the attention of a driver who hasn't seen you and may hit you or possibly kill you if you don't get their attention. I mostly ride on open roads, and through small towns, and I don't have one, but I definitely would get one if I cycled in city traffic.
A few years ago, when I was driving a bus, I had to stop behind a car that was trying to turn into a petrol station. He couldn't get in because it was very busy (arguably, he should have carried on and looked for another one, but that's not the point of this story). The driver in front of him decided not to wait and started to reverse without checking behind him. The second car driver tooted his horn, but he just carried on reversing, so I hit the air horn on the bus. It's amazing the attention an air horn gets! Everyone stopped and looked round, including the reversing driver. I prevented a minor bump that day, but if the car had been a motorbike, or even a cyclist, it could potentially have been very serious.
That's what air horns are for - not for "making a point" once the moment of danger has passed.