The answers to your questions depend on what you think is the purpose of a published book review.

In my discipline (mathematics), reviews of research monographs that appear in journals usually have an opening paragraph describing the book under review and a couple of closing paragraphs summarizing the contents of the book, judging the quality of the book, and sometimes listing the errors found by the reviewer. In between those paragraphs are pages and pages in which the reviewer usually gives his or her personal view of the subject of the book. That is, book reviews are seen as excuses for a reviewer to write exposition about a subject he or she enjoys. I think it's a little strange, and rarely interesting, but that's the norm.

I agree with the sentiment I once read that it's a waste of time to review a bad book, especially a mediocre one. A book review should, in my opinion, either steer the reader towards a book worth reading, explaining why the book is worthy and important; or, in cases of books published with a lot of fanfare (e.g., Wolfram's A New Kind of Science), discuss the merits or lack thereof of a book of which the reader is already aware. Why waste the time of reviewers and readers trashing a book no one will hear of otherwise? Just let such books die deservedly in obscurity.

A notable exception is an entertaining review of a truly wretched book. That's fun to read, but only if the reviewer is sufficiently talented to pull it off.