By ignoring a motorist who, say, honks at me while I'm legitimately merging left into a left turn lane, I'm giving him subtle permission to do it again? So instead of ignoring him I should be what, moving aside, flipping him off, yelling at him? That would be more effective? I don't think so.

Now, if there is an opportunity to talk to him about it (because that's always so fruitful ), I might take it. But I'm thinking about situations where there is no opportunity to do that.