According to an article on Slate.com:
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?
Nope—or that's my take, anyway. Now, I'm the first to admit—before you Google and shame me with a thousand examples in the comments—that I'm no saint when it comes to the em dash. I never met a sentence I didn't want to make just a bit longer—and so the dash is my embarrassing best friend. When the New York Times' associate managing editor for standards—Philip B. Corbett, for the record—wrote a blog post scolding Times writers for overusing the dash (as many as five dashes snuck their way into a single 3.5-paragraph story on A1, to his horror), an old friend from my college newspaper emailed it to me. "Reminded me of our battles over long dashes," he wrote—and, to tell the truth, I wasn't on the anti-dash side back then. But as I've read and written more in the ensuing years, my reliance on the dash has come to feel like a pack-a-day cigarette habit—I know it makes me look and sound and feel terrible—and so I'm trying to quit.
The problem with the dash—as you may have noticed!—is that it discourages truly efficient writing. It also—and this might be its worst sin—disrupts the flow of a sentence. Don't you find it annoying—and you can tell me if you do, I won't be hurt—when a writer inserts a thought into the midst of another one that's not yet complete? Strunk and White—who must always be mentioned in articles such as this one—counsel against overusing the dash as well: "Use a dash only when a more common mark of punctuation seems inadequate." Who are we, we modern writers, to pass judgment—and with such shocking frequency—on these more simple forms of punctuation—the workmanlike comma, the stalwart colon, the taken-for-granted period? (One colleague—arguing strenuously that certain occasions call for the dash instead of other punctuation, for purposes of tone—told me he thinks of the parenthesis as a whisper, and the dash as a way of calling attention to a phrase. As for what I think of his observation—well, consider how I have chosen to offset it.)
Perhaps, in some way, the recent rise of the dash—and this "trend" is just anecdotal observation; I admit I haven't found a way to crunch the numbers—is a reaction to our attention-deficit-disordered culture, in which we toggle between tabs and ideas and conversations all day. An explanation is not an excuse, though—as Corbett wrote in another sensible harangue against the dash, "Sometimes a procession of such punctuation is a hint that a sentence is overstuffed or needs rethinking." Why not try for clarity in our writing—if not our lives?
It's unclear—even among the printing community—when the em dash came into common usage. Folklore—if you're willing to trust it—holds that it's been around since the days of Gutenberg but didn't catch on until at least the 1700s because the em dash wasn't used in the Bible, and thus was considered an inferior bit of punctuation. The symbol derives its name from its width—approximately equal to an m—and is easily confused with its close cousin the en dash, used more frequently across the pond, but here meant only to offset sports scores and the like. The em dash isn't easily formed on computers—it requires some special keystrokes on both PCs and Macs—and so I will admit that at least some of my bile comes from, as a copy editor, endlessly changing other writers' sloppy em-dash simulacra (the double dash, the single offset dash) to the real thing.
Perhaps the most famous dash-user in history—though she didn't use the em dash conventionally—was Emily Dickinson. According to the essay "Emily Dickinson's Volcanic Punctuation" from a 1993 edition of The Emily Dickinson Journal—a true general-interest read!—"Dickinson's excessive use of dashes has been interpreted variously as the result of great stress and intense emotion, as the indication of a mental breakdown, and as a mere idiosyncratic, female habit." Can there really be—at the risk of sounding like a troglodyte—something feminine about the use of a dash, some sort of lighthearted gossamer quality? Compare Dickinson's stylistic flitting with the brutally short sentences of male writers—Hemingway, for instance—who, arguably, use their clipped style to evoke taciturn masculinity. Henry Fielding apparently rewrote his sister Sarah's work heavily to edit out some of her idiosyncrasies—chief among them, a devotion to the dash. In Gore Vidal's Burr, the title character complains—in a charming internal monologue—"Why am I using so many dashes? Like a schoolgirl. The dash is the sign of a poor style. Jefferson used to hurl them like javelins across the page." So is the rise of the dash related—as everything seems to be these days—to the End of Men? (I kid—calm down.)
More likely, it's the lack of hard-and-fast usage rules—even the AP's guidelines are more suggestions than anything—that makes the dash so popular in our post-sentence-diagramming era. According to Lynne Truss—the closest thing we've got to a celebrity grammarian, thanks to her best-seller Eats, Shoots and Leaves—people use the em dash because "they know you can't use it wrongly—which for a punctuation mark, is an uncommon virtue."
So, fine, the em dash is easy to turn to—any port will do in a storm. But if you want to make your point—directly, with clarity, and memorably—I have some advice you'd do well to consider. Leave the damn em dash alone.