Whoever coined that particular phrase couldn't have done much reading!
What the cover looks like doesn't matter to me when it's the latest book of a well-loved or favourite author. I'm not too bothered about the cover or the title, or even the blurb, because I know I'm going to enjoy reading it. And I'm not too bothered if it's an author I've never read before but the book is either highly recommended or has generated a lot of comments. Then I simply have to see what all the fuss is about. Take Fifty Shades, for instance, (didn't do anything for me), or Gone Girl (loved it) - had to read both of these because of the hype, and I don't think I could tell you what's on the cover of either of them.
It's a bit different when it comes to an unknown author (unknown to me, at least). I browse by both cover and title (and thickness or word count, but I've talked about that weird fetish of mine before), then blurb.
Covers are important - I keep stressing this, don't I? But if I saw a book with a fighter plane on the front I wouldn't even bother looking at the back. I would assume it was about war, and that's not a genre I'm into at the moment, though I've read my fair share of them in the past.
What's brought all this up is my finding a novel about werewolves which had a George Eliot pastoral garden scene on the cover.
Needless to say, I didn't bother to read it. So, yes, I'm definitely guilty of judging books by their covers.