I hope this horn sound is a quick "chirp" and not a heavy handed horn blast like all those minivans in the parking lot that love scaring the hell out of people walking by!
I agree. Personally, I think ANY use of the horn for confirmation is anti-social. Horns are obnoxious, and we do not need any more horn blasts in parking lots, etc. In parking garages, the horn is extremely unpleasant. A flash of the lights is sufficient, which is what my Model S does now and is all anyone should need, IMHO.
I surely hope this "feature" does not become common. Teslas are known as quiet cars. Let's not start adding unnecessarily to the cacophony of daily life....