Oh I use yelling too. Thing is I don't always ride with a whistle at the ready in my mouth. I put it there when I know I am coming up to busy intersections, or places of concern. So otherwise, if someone or thing surprises me I braek out the vocal cords.

All in all though the whistle is many time louder than a yell, and it won't get lost as easily in the throng of noises on the street. People respond to a whistle immediately I think because they associate it with the authorities. Which is probably why it is illegal in some places.