Design is all about making decisions.
Because Seth’s specialty is marketing, his advice to make sure every graph tells a story may not always be necessary for a researcher to implement. In research settings, graphs are often used to show details that are pieces of the big picture, whereas in business, the graphs are the big picture.
Finally, Seth is also a fan of being conservative, much like Rob Sawyer is with book layout. Following established conventions helps the viewer. And if you think following conventions limits your creativity, ask if Shakespeare found the rigid rhyming scheme of sonnets too restrictive.
Additional: A follow-up at Charts.