Noreen Malone in Slate:
According to the Associated Press Stylebook— Slate’s bible for all things punctuation- and grammar-related—there are two main prose uses—the abrupt change and the series within a phrase—for the em dash. The guide does not explicitly say that writers can use the dash in lieu of properly crafting sentences, or instead of a comma or a parenthetical or a colon—and yet in practical usage, we do. A lot—or so I have observed lately. America’s finest prose—in blogs, magazines, newspapers, or novels—is littered with so many dashes among the dots it’s as if the language is signaling distress in Morse code.
What’s the matter with an em dash or two, you ask?—or so I like to imagine. What’s not to like about a sentence that explores in full all the punctuational options—sometimes a dash, sometimes an ellipsis, sometimes a nice semicolon at just the right moment—in order to seem more complex and syntactically interesting, to reach its full potential? Doesn’t a dash—if done right—let the writer maintain an elegant, sinewy flow to her sentences?