This is a big practice many reviewers embrace that drive me nuts as an author and a reader. How can you review a book without finishing it? Not finishing it is passing judgement on something you are not familiar with.
Missing the Whole Picture
Not finishing a book means you only get a piece of the larger picture. The first chapters might be weak. If you stop reading too soon, you miss out on better writing that comes later. A bad review is then not on the full novel. It is only on a tiny portion. That means you are misleading anyone who reads your review.
A review is a communication to other readers. You are telling other readers what they need to know about the took to aid in their decision on whether to buy it or not. Not knowing the full story prevents you from giving a truthful review. I wanted to read a classical piece of Russion writing. I chose Crime and Punishment by Fyodor Dostoevsky. I thought it was horrible as I got past the first few pages. It wasn’t until the last thirty pages or so that I found myself enjoying it. If I had stopped reading it early on, I would have missed out on a great story. I wouldn’t have had the whole picture.
From a Reader’s Standpoint
Think of it as a reader and not as a reviewer. You want to know about the entire story, not just the first five pages. Yes, some start off slow, but in the end it was wonderful. A reader wants to know the full truth.
Give the reader everything they need to know in making reading decisions. Let them know the true development of the plot and the characters.
Possibly Only Exception
There has only been one exception I have found to this rule though I still stand by the fact that a reviewer should read the whole book.I started a book that had been heavily promoted. I struggled from page one. Page two was worse. This book was published by a traditional publisher. Despite that, it read as though English was a second language and no proofreading had been applied to it. I managed to get through one chapter before I had to call it quits. I loaned the book to a friend who couldn’t finish it either. In this situation, I did write a review on it so others would not waste their money on a poorly published book.
I wish I had finished it. I tried a few other times and couldn’t. It was truly that bad. In my review, I did state that I couldn’t finish it and why. That way a reader can decide to push on if my issues don’t bother them.