I have a hard time staying focused on lectures unless I'm taking notes. There really is something about this process that helps me to focus and organize the material. If I find myself in a class where I'm scribbling as fast as I can just to keep up, it's pretty much hopeless as I've lost the thread of the lecture and the notes will be of no use. Professors need to better organize lectures so this doesn't happen.
Another thing, the professor may choose to cover material that isn't in the textbook, so notes can be essential then.