Never seen the point. Bells work 66% of the time
[1] on shared paths. Incoherent yelping is all I have time for for alerting lemming pedestrians, though the Koolstop squeal often helps.
Presumably the benefit of an airhorn is that motorists are more likely to hear it. I'm not sure how helpful that is, as assuming they realise that horn noise is coming from the bike, you don't want to rely on them reacting appropriately for your safety. At which point it's only useful as a means of escalating conflict after an incident.
The velomobileists tend to have electric horns. I can imagine they're slightly more useful when a bell would be muffled, and you're moving at a speed that people don't necessarily expect.
The only cyclist I know with an airhorn is a full-on
Captain Dashboard type, with a bike adorned with all sorts of homebrew visibility enhancements. He seems to ensure this gets regular testing by means of some highly dubious road positioning.
[1] On the basis that a third of the time they work as intended, a third of the time they're misinterpreted as "GETOUTOFMYFUCKINGWAYBLOODYCYCLISTCOMINGTHROUGH", and the rest of the time they're not heard.