On a related note, hand gestures are usually for the benefit of the speaker, not the audience, for a very similar reason. I tried holding hands with a girl during a classroom discussion once, and I couldn't even begin to speak without bringing my hands out.

I talk to myself a lot. I was an only child, so I never really got socialized against it, and often didn't have anyone else present to talk to. However, this usually happens when I'm idly woolgathering, not when I'm working on a problem. Problems require writing!