There's this whole culture of "don't be mean!" and "writing books is hard!" around book reviews in some circles, which as you say, misses the point. "Don't be mean!" is fine advice for people (which is why personal attacks or insulting the author as a human being is right out), but it makes no sense for books, any more than complaining about a slightly off meal eaten at a restaurant because the chef's feelings might be hurt makes sense.

But then, I'm still having trouble (having not read the books yet) with the fact that Blackout / All Clear were considered as a single book by the Hugo folks, simply because the author felt they should be. Haven't lots of authors felt the publisher split their books unreasonably? And hasn't what the author felt generally been irrelevant before, and only the book-at-hand relevant, in isolation from same?