I think if people took this book very seriously, and did the exercises, they would actually develop their drawing skills very far. I like that this book doesn't superimpose any kind of set style (although it gives a lot of methods that make it seem like so).
I've picked up a few things from this book while reading it. I think the most important thing it emphasizes that looking is just as important as drawing. I personally think observation really is that important: it lets you freely use the information you know in your work, and conclusively make it more real.