Through a passage vs. Topical

I saw a picture similar to this one in a friend's textbook. I thought that it graphically made a very important point: preaching through a passage, if done well, is most likely going to present the original author's point. Preaching topically, which often boils down to picking thoughts from different authors, is most likely going to present the preacher's own point instead of the original authors' intended points, which is a problem since ultimately, when we teach or preach, we are to present's God's truth, not our ideas.