There are some people who subscribe to the "if you don't have anything nice to say, don't say anything at all" school of reviewing. I am not one of those people. I've written my fair share of book reviews, a few of which were less than favorable (the word "scathing" has been suggested). Although I generally ended up reviewing books I disliked simply because they were assigned to me by an editor, I've occasionally written reviews of books I detested because I felt they were so flawed that they deserved public treatment rather than silence. And that's one of the principle reasons why, à mon avis, the negative review should be written: to help correct the bias generated by solely positive reviews, since such reviews are oftentimes met only with silence by those with dissenting opinions. The fact that we so often don't finish books we dislike only compounds the problem.
More importantly, though, is this: rarely do we question a positive or even neutral response to a book, but as soon as someone indicates that they didn't like—or even flat-out hated—that book, we immediately want to know why. What didn't the reviewer like? What worked and what didn't? Was it the writing? The pacing? The characters? The list is endless, and a well-written negative review is often just as illuminating as a positive one, if not moreso. Think of it this way: you rarely question what it is that makes your car or refrigerator or laptop work while it's running properly, but as soon as it breaks, you want the nitty-gritty on what's gone awry. The same goes for books: we don't know what makes good fiction until it's missing.
There are a couple of caveats here, though, and I hope they're somewhat intuitive. First, the review should be about the book and not about the author. As I've said before, a bad review is (or should be) an expressed opinion of a given book, not an indictment of the author's character. To cross the line and malign the author of a book for what he or she has written—even if it's clear from the writing that the author is a raging misogynist, a blatant racist, or worse, a member of Congress—is beside the point. All reviewers (myself included) have strayed from this point from time to time, but it's important to remember that book reviews are exactly that: book reviews. Not author reviews. Not ideology reviews. Book reviews.
Additionally, the review needs to explain why the reviewer didn't like the book. Going back to my earlier car/fridge/laptop example, it doesn't do you any good to simply know that your property is broken; you want to know what, exactly, is wrong. You can't fix something without knowing what's wrong with it, and authors can't avoid their previous pitfalls (or the pitfalls of others) without knowing what reviewers find lacking in their work.
I'm curious to know what you think, though, auteurs and... auteuses? (My French isn't as good as it used to be.) Do you think the negative review has a place in contemporary criticism? How would you react to a negative review of your own work?