As a writer of many, many (often uninteresting) books, I can confidently say that a 3-star review is better than no review, especially if a book has few (or no) reviews. Why? It sells books! (Counterintuitive but true. Maybe the fact that other people bought the book makes someone feel more comfortable in joining them.) So reader and potential reviewer, we appreciate the effort to say something, anything, really, and it doesn’t need to be particularly intelligent.