To quote Kevin Spacey's disarmingly frank line from American Beauty: "I just want to look good naked."
And that's the trick of the bikini - you're not wearing much, so everything's gotta look good. You can't cover up those flabby upper arms and hope the push-up bra will be distracting enough. You can't wear waterproof mascara and assume people are looking at your face. Everything is up for dissection by the public.
Same goes for your book.
The cover and first chapter are important, like your general silhouette. You might be able to reel them in, but are they gonna get closer and go for the casual nod instead of engaging? What if Chapter Two is the equivalent of starting a conversation to find out you've got bad breath?
Too often I hear writers say, "Yeah there's a downswing here but the next scene really picks up." Or, "I know there's a huge info dump at the beginning but if you can get past that, it's totally awesome." Right. And the obese chick with a good personality gets all the guys on the beach.
Your book is going to be naked. Every page is going to be turned (hopefully). Every word will be exposed to an eyeball.
And you can't very well say, "Do me a favor and read this next bit in the dark."