Unless the goal of the review is to talk about the interesting parts of a book and how they damage or point to flaws in the genre. "Interesting" is not always the same thing as "good", and it's certainly not the same thing as "not worth examining critically". Sometimes a book's flaws are the most interesting thing about it.
montoya makes the counterargument:
Most of the bad books I read are bad in boring ways. Too dull, characters I don't care about, cliched, whatever. Very few are bad in interesting ways.
Yes. As I think I said in a comment elsewhere, one of the types of review of a bad book that we will sometimes run is one that points to a larger failing.
Of course, discussing the weaknesses of good books usually points out those larger failings as well--it's a rare book that has a flaw that nothing else in the world shares.
("We" of course is NYRSF; we were not the subject of the original discussion, as nearly as I can tell--it was all done without proper nouns-- but our stated policy is that we desire reviews "which reveal the strengths and weaknesses of good books". These tend to be positive reviews, although sometimes they're not.)
Interestingly, the only other venue for which I've ever written reviews professionally--Games Magazine--also only publishes reviews of good games. There, it's definitely because they only have room to discuss four board games per issue, so the editor doesn't want to waste space on bad games.