Janni Lee Simmer, kindly linking to my "how I do PW stuff" series, noted: "I often hear writers wondering why anyone would take the time to give negative reviews, save out of meanness--but I think there are other reasons not all reviews should be positive, too, and it's good to hear someone being articulate about the fact."
As I commented to her, I don't think I've ever heard anyone question the need for negative reviews. I have certainly heard specific instances of them questioned--and not always by the person whose work is being reviewed--but I'm surprised by her suggestion that some have issues with the entire class. (If you're one of those people, whence comes your broad dislike of negative reviews? Please do explain in comments; I'm curious.)
There are plenty of ways to justify the writing of negative reviews (other than "It's fun!", which is often true but also insufficient). From the perspective of the reviewer and the reviewing publication, it lends credibility: much as I can trust that my husband really loves the second scarf I knitted him because he was honest about not liking the first one, readers who know that a publication is willing to give negative reviews will trust that when that publication does give a positive review, it's for good reason. From the perspective of the reader, it's helpful: the market is glutted with beautifully packaged, well-blurbed books, and readers need to know what to avoid as well as what to seek out. From the perspective of the writer and the publisher, it's educational: they may be too deeply involved with a book to really be able to predict how readers will react to it, and an honest review gives them valuable information about how to improve the writing and marketing of future books.
Any one of those reasons would be sufficient to justify the practice, I think, and the sum of the three makes it imperative. No one gains when a publication or reviewer only gives positive reviews.
It should (but probably does not) go without saying that none of this is a justification for a cruel review. As with any communication--and it's important to remember that reviews are fundamentally a form of communication--criticism should be delivered with at least as much tact as honesty. It's kinder to the author and publisher, without whom the reviewing profession would not exist; they have taken the leap of submitting a book for review, and that bold move deserves to be met with kindness as well as frankness. (If we bring self-interest into it, in the inbred world of genre publishing it's entirely plausible that a given reviewer might end up working for that author or publisher someday, so that alone is probably a good idea to refrain from getting too snarktastic.)
It's also not a justification for nonspecific complaining. I recall something about Graham Sleight and John Clute agreeing to avoid certain terms in their reviews--"disappointing", maybe, or "unsuccessful"? Graham, please jog my memory here--because they just don't convey enough useful information. "This was disappointing" is just a nicer way of saying "This sucks", and "This sucks" is not a review. A measured, detailed negative review, on the other hand, is a service to the readers and lets them make up their own minds. If you say "The focus is on the complex characters at the expense of the worldbuilding" some readers will immediately run as far from the book as possible, while others, who couldn't care less about worldbuilding but love well-developed characters, will run to pick it up.
I've seen some reviewers approach reviewing as an exercise in getting readers to agree with them. That seems to me more like an exercise in egotism. I believe the reviewer's job is to inform without manipulating, insofar as that can be managed; all the more reason to avoid harsh words that manipulate readers' emotions rather than helping them to make informed decisions.
Having hopefully conveyed my opinion in an informative but non-manipulative fashion, I'm off for the next week. I have a great deal of work to do tomorr--er, today (how did it get to be past midnight?) and Wednesday, and then we're off to Arisia. I hope those of you who also plan to be there will introduce yourselves! Josh and I are running the green room, so I have no doubt we'll see many of you at least in passing. We come back the night of Monday the 21st; blogging resumes Tuesday. I hope the week treats you all well.