If you can see a motorist who obviously hasn't seen you, it's far better to slow down than to use a horn. The motorist might not hear the horn and may continue, and you'd be dead or injured. Speeding into a situation and relying on a horn is, in my view, suicidal.
Relying on a horn rather than on brakes is false security.
Most handlebar layouts make it a choice between braking effectively or horning.
This is what I was getting at in my post. Ian and GC explained it better.
In a SMIDSY situation with someone pulling out on you, if I apply the brakes and stop I have taken control of the situation, and have ensured my own safety. I'd much rather this than continue my set course and pace relying on the motorist to hear (and yield to) my horn just because it's my "right of way".