I think this is a completely valid argument... but if you think about it... the people that are concerned enough to add an extra noise to their car to warn pedestrians they are coming, are probably the only people who actually pay attention while they are driving.
I really like MetroMPG's idea of adding a tiny or bicycle horn for close-up honking. I find myself in the position all the time where I really want to honk at the person but I end up not pushing hard enough on the button and I end up not honking at all.
I actually have a Honda spree horn hooked up to my car right now because one of my horns broke, but if I could isolate it, that would be awesome. Thank's for the idea!!!