As a writer, I understand how important reviews are to the process. They not only help boost the author's confidence, but double the effort that goes into every nuance. Reviewers can make or break an author, depending on the feedback and the resiliance. A bad review could put a halt to an entire story, or push enough that it is completed in record time.
I write lackluster reviews. I forget grammar, and throw spelling out the window. I loose focus after a single sentence. I know what parts I like, or what scenes I thought were silly. I know when a character gets on my nerves, or when I feel like cheering for another. When it comes down to execution, writing something meaningful, I fall short. So, to save face, I don't leave a review. I'm lurker, I guess. I just don't see the point of leaving a crap comment.