Multitasking isn't always flawed.

One of the most well-read person I know recently revealed to me that the hundreds of books he read over past few years were actually mostly audiobooks - because it turns out that most of his dayjob is physical, low-cognitive-demand kind, and pairs perfectly with audiobooks, basically keeping his brain engaged while his muscles do the repetitive work.

What you're saying is probably correct for some percent of readers, but I'm not sure how big the effect actually is. Personally, I listen to books that I never would have sat down to read in the first place, and the time it takes up isn't competing with reading (doing chores, driving, etc.).