You can train yourself to talk while drawing - a few words might keep you from having to shoot.
In some situations, you might as well. In other situations, you might want to keep your mouth shut and avoid drawing attention to yourself.
For example, if someone is walking through my home at night with a weapon in his hand, (more akin to the situation at hand) he's not going to know what hit him. If someone confronts me on the street with a knife or baseball bat 20 feet away, I might yell "drop your weapon" as I'm drawing.