The usual insufferable tweedwads argue that literary criticism is a genre unto itself, its value residing not in the appraisal of the book so much as the context, scholarship and thematic exploration offered by the critic. Uh-huh. Sure. Go ahead, Margaret Atwood — make this about you.
The other silly argument is that a positive review is rendered meaningless if there is no possibility for a negative one. Oh, really? Ever see a hyperlink?
The single kernel of truth in their justification — "Why waste breath talking smack about something?" — is surrounded by so many acres of idiocity I'd need hip boots to wade over there. And it turns out that one kernel is, on closer inspection, a withered husk.
Why waste breath talking smack about something? Because sometimes that's the only way to find out what's wrong with it — by pointing out the emperor has no clothes, and by doing so in language that leaves nothing to doubt. It shocks people out of their aesthetic complacency.
Some of my favorite things have been attacked in precisely this way, and after I set aside my initial bursts of anger (how dare they attack My Critical Darlings!!!), I realized my favoritism might well have been just that: an emotional defense of something which didn't really deserve it. It works the other way 'round, too: some things I've previously dismissed as inconsequential I saw in a new light after someone was motivated to make a passionate and informed defense of it. (It's hard not to get swept up in someone else's emotional crusade about something, positive or negative.)
My point is that going all-out two-fisted on something isn't a casual decision. It should be reserved for when there's a major bandwagon to overturn: e.g., Devin Faraci standing his ground that the conclusion to Man of Steel was thunderously overblown and in fact defeated the movie's own purpose. (I agree in principle, but not to the degree he does.) But there are times when you do need to roll up your sleeves and ball your fists.