Why? It seems to be for the sake of effect, to signal that a hardcover is particularly distinguished. I imagine that it's thought to look old, and here the implicit metaphor shows its falseness. Once upon a time in the Anglo-American book world, a customer sometimes bought the pages of a book or pamphlet as a stack of printed but loose fascicles, and then had them trimmed and bound himself. (New books in France are issued as paperbacks on this understanding, though I doubt that very many French readers still get around to binding anymore.) To leave the outer edges untrimmed evokes this era, when a customer could choose to leave one edge rough . . . if he were pretentious and not in the habit of actually reading his books.
It's wretched nostalgia, and it should be stopped. All binding is centralized today, and so no customer chooses deckle edges anymore, and no one can opt out of it, either, except by waiting for the paperback. Deckle edges absorb and retain dust with fantastic efficiency. But the truly demonic thing about them is that they turn a book into a trick deck of cards---the sort where if you flip the cards over one by one, you see all suits, but if you riffle along one edge, you see only clubs or only diamonds. If you riffle through the pages of a deckle-edged book, only certain pages flip open to your eye. Should you be hunting for a quote that happens to appear on a page whose width is not a local maximum, you're out of luck. You can try putting your thumb along the clean-cut top or bottom, and you'll get a little functionality this way. But not much. It becomes hard even to figure out what chapter you're in. O publishers of America, stop it. If you must deliver hoity-toityness, how about a nice daub of gilt on the tops of pages, or a dash of blue paint? Or you might sew the signatures together instead of gluing them. A frisson of "quality" always comes over me when I see white thread. Of course that would cost money, and I suspect that deckle edges are as cheap as they look.
