There are lots of different reasons for writing a positive review of a particular book. Maybe you like the book and want other people to read it. Or maybe you don't like it that much but it was okay, and you like the author. Or you owe the author because the author invited you to his wife's gallery opening and you drank an awful lot of the free wine, or you like the author's agent and you want the author's agent to like you, or maybe you work for a bookstore and your job is to sell books, dammit, not put people off books.
But why write a negative review? You might have a Consumer Reports kind of conscience, and don't want people to be disappointed or waste their money or be somehow permanently damaged by a book. That would be very thoughtful of you. But if you do trash a book, it means you're doing something that might thwart a writer's career -- and if you're a writer yourself (as many if not most reviewers these days are) then you're sabotaging a colleague... or a competitor. You will likely be suspected of jealousy, or worse.
(And it's true that some negative reviews come across as positively gleeful. A rare few are so fantastically venomous they make me wish I knew the stories behind them, even compel me to invent some. Did Writer A kiss Reviewer B's boyfriend behind the bowling alley at Yaddo? (Does Yaddo have a bowling alley? It ought.) Was Reviewer C in the constant shadow of Writer D at Prestigious Workshop E?)
Most of the time, negative reviewing is a quagmire for the critic as well as for the writer (though usually a delight for the reader). No one will call you up to thank you for that awesome skewering in last week's Binghamton Gazette. Writers have photographic memories for these things, and who knows if one day you'll find yourself abandoned at a cocktail party with the victim of your hatchet job, or -- and this is the real, secret fear behind all those middling, non-committal reviews -- maybe one day your career will be in that writer's hands, and that writer won't be in the mood to grant you mercy.
So for all these reasons I periodically decide that I'm against negative reviewing. I've been on the receiving end, too, and it hoits: you think, Other people liked my book, lady, so who makes you the big fat boss? Struggling writers need our help, right? Right?
I'm not so sure. A jealous, spiteful, insincere rant is as useless as a log-rolling softball blurb, albeit much more fun to read. But a good negative review kicks all of us in the pants. A good bad review is an attempt to keep criticism from turning into marketing, in spite of great forces pushing it in that direction. Book reviewing should be a conversation, not a sales pitch.
Those reviewers who studiously ignore the books they don't like aren't doing anyone any favors; they're pretending the world of literature is a big garden full of Easter eggs instead of the mess of complicated, controversial, and engaging arguments that it really is. Criticizing a fellow writer in a public venue is not easy and comes with actual risks -- suddenly, some people who didn't even know you before now hate your guts. But somebody's got to do it. So, on with the chainmail!