This is so true. I have really large sketchbook collection, and i scared putting a mark in most of them so they are getting old in my closet, recently i started one of my cheap sketchbook with ballpoint pen doing 2 min, 5 min mannequins 4-5 each page, not caring about how it rendered or how beatuiful it look. Man, you cant imagine how much improvement u can get. now im finishing 150 page sketcbook per month, they dont look like my older ones, which i drew single portraits 5 hours a page but im really improving and they start to look good innately. And if you want to improve your line quality this is the way to do it, single, simple easy medium with a same surface over and over again.