Everybody who writes reviews, myself included, loves to read a compelling book and then have the opportunity to tell others why we think it's a worthwhile read. But as often as not we are asked to review a book that is poorly written, boring, or simply rambles on with no clear direction. Many of the writers chiming in today said they have no issues with writing a scathing review. Some reviewers take the time to contact the author and let them decide whether they want a bad review or nothing at all. (Surprisingly, to me anyway, many writers prefer a bad review rather than no review.)
For me, it's a bit of a toss up. I will not write a review that completely dishes a book. If I found enough positives that I can give a balanced review, pointing out as many positives as negatives, then I will write the review, regardless of what the author wants. But if there were no positives, and I've read several books that fell into that category, then I will simply not write a review.
It's more of a personal thing. I don't like writing negative reviews. Also, I know what it feels like to be on the receiving end of a bad review, and I have no wish to cause a writer that anguish.