The advanced drawing students I've been working with have been taught the value of tone and have learned the lesson well. I am now trying to encourage them to explore line more fully. A common misconception is that tone is the only way to create volume and mass. Tone certainly helps but the line drawing itself, when understood and used well can do much of the work. To illustrate, I did this demo for my class.
This final version shows the integration of line and tone that I tend to like in a drawing. Here is a wonderful example of what I'm talking about, a drawing by Jerome Witkin.