You enter a bookstore. The front shelves are almost always occupied by “bestsellers” - the ones that latterly everyone seems to be reading and talking about. Further you encounter the “classics” - the ones who withstood the time and commonly agreed to be of importance in their contributions to humanity or history. Further away are domain experts arranged by thematic shelves - the typical pundits of the subject matter. But if you venture deeper into the depth of the bookstore, or browse outside the “eye level shelves”, you will find “the others” - the ones who never made the cut.
So you become adventurous and decide to give a chance to the unknown book, not previously reviewed by your friends or seen on the Amazon's “recommended”. However after a couple of pages (or chapters) you realize you are not enjoying it at all. Reasons canary from distaste of the writing style (the language is too complicated/simplistic/deficient) to the subject (the topic is not covered/irrelvant/lukewarm/complicated/difficult to grasp). And so you place the label “bad book”. Sometimes we don’t even go through the mental exercise to elaborate to ourselves why we don’t like the book, we just easily toss it the aside (whether with or without intention to come back to it) and move on.
Both hedonistic and utilatarian booksworms would probably agree that it is the right decision, since reading is a pleasurable activity and should be spent on good books. I, nevertheless, want to proclaim that it is our duty to read and most importantly finish “bad books”. Let me start my argument with conscious selection of bad books. Venturing outside of popular reading list can be as advantageous as diverging from your favorite flavor of ice cream (“are you sure you like vanilla? how about raspberry-mint”). It may seem like a very self-evident argument, but I am writing this in 2019 - in the time of the rule of content bubbles and recommendation algorithms that rarely let us break outside of similarity patterns. In the past browsing in bookstores and libraries gave one a sense of serendipity, stumbling on interesting cover, the unexpected title, the new section. Unfortunately amazon rarely gives you an overview of their complete database with a content marker "You are here". Hence frisky reading choices are mandatory to leave the realms of self-refferntial knowledge loop.
An unexpected reward of reading "bad books" is the perspective on appreciation of the good ones. After making a detour into irksome topic or genre, it is always a true delight to come back to your personal favorites. But even better, they feel more nutritious, your want to savor them longer, you pay closer attention to the words used. Whether we return to olf-favorite authors, or discover new ones, the process of reading the books that correspond to our taste feels more acute and it feels like ideas penetrate deeper. Hence I can not imagine enjoying my favorite written works without the exposure to underwhelming and somethimes quite commonplace literature. In a way it sharpens your taste.
In case none of any of the points above convinced you, my last justification to finish the book is the benefit of discipline and the responsibility of finishing things. By finishing the book we allow the other person finish talking without interrupting: regardless whether the person is a sage or a fool. By finishing the book we demonstrate consideration and ability to entertain ideas we do not share. In the end the author did put the time to convert his thoughts into sentences, so the least we can do is to give a little bit of our time to contemplate on his/her musings. While reading a "bad book" I like to conduct an imaginary dialogue with author questioning, but I do think it's mandatory to let them finish the sentence...
Not every book we read should be a page-turner, but some that we come across can demonstrate the spectrum of variation in human writing. So be adventurous and pass by those bestsellers and classics, heading to the "un-reviewed" and the "un-starred". In the end you are are around 50,000 words away from considering something you have not considered before.