why read a bad book?
When a writer reads a bad book, they have the chance to address why it is a bad book. What does the book do poorly? How does it do these things poorly? This offers them the chance to understand the flaws in their own writing. Formulating why something is done poorly in one book can cause a writer to realize they're making the same mistake. In other words, reading bad books enables a writer to gain a new perspective on their own work.
A writer must counteract bad books with good books. If title1 does X poorly, how does title2 do X well? What has title2 done differently? How did title2 circumvent title1's mistakes? One could take this a step farther---use what one has learned from title2 to figure out how to 'fix' title1.
A reader who does not write may temporarily benefit from reading bad books. They develop a taste in literature by figuring out what they like and dislike. Reading bad books is a step towards this. However, they may fall victim to only reading bad books. This limits their ability to grow as a person. Readers are permitted to continue to read bad books iff they understand how bad books have their time and place. They must also counteract bad books with good books; how else will they expose themselves to new ideas and think about the world?
Reading bad books can help a reader have a better appreciation for good books. They are able to compare bad books to good books; in doing so, they may have a better understanding of writers, and be more able to appreciate the effort good writers put into good books. Then they may be able to understand how a writer became a good writer---see the writer's evolution and be able to explain how their writing has changed.