I agree with Brendan Wolfe that book reviews can be "tools that teach us how to think about books, how to read books, how to judge books." I also agree that a negative review can be used as such a tool just as readily as a positive review--but only just. If a negative review needs to be more than "snark," a positive review needs to be more than simple praise. Reviews ought to go beyond being simply "consumer research tools" (quoting Brendan), whether they result in readers seeking out a book or in leaving it alone.
(I can't agree with Brendan, however, that Dale Peck in particular is a reviewer who "writes well, makes interesting & thoughtful arguments, advances the discussion, advocates for the relevance of art," even if his judgment can ultimately be called into question. In my opinion, Peck is, snark aside, a dull writer who rarely offers thoughtful insights that aren't just bluster, "advances the discussion" only in the sense Scott Esposito has in mind when he complains about reviewers whose reviews are ultimately "more about [the reviewer] than the book," and advocates only for himself.)
However, I still question the utility of negative reviews that are not focused on a flaw that can be used by the reviewer to illuminate some larger issue relevant to literature as a whole. If, for example, a first novel illustrates an endemic problem the reviewer finds in first novels in general (or recent first novels, at least), then a negative review can be entirely justified, but I don't see the point in trashing a writer's first effort just for the sake of registering one's disapproval. (Or even books by established writers. Recently I've had the displeasure of reading a series of books by otherwise well-known writers that, unfortunately, I didn't like. In some cases, I couldn't even finish the book because of my extreme boredom with it. But I haven't posted about them or attempted a review of them simply to make my indifferent response a part of the public record. Here, William Gass's advice that an unworthy book will be "quickly forgotten" if we "simply not speak its name" seems appropriate.) I also think that negative reviews of books or authors whose reputation is (in the reviewer's opinion) unduly inflated are perfectly acceptable, if the reviewer is able to again make the question of "how to read books" and "how to judge books" central to the review.
If, however, I had to declare which kind of review or critical essay, the censorious or the laudatory, has in my experience more effectively allowed me to discharge what I consider to be the critic's most important tasks--to describe and evaluate the various ways writers can exploit the possibilities of fiction or poetry--I would say it is the latter. I feel I am doing some kind of service (or attempting to do so) toward the maintenance of these possibilities by calling attention to writers and works that embody them in compelling ways, more than I am usually able to by focusing on what's wrong or what isn't there.