Maybe—just maybe—it comes from the current tendency—in a world of constantly bleating cellphones and multitudinous—even bewildering—distractions—but it seems to me—though this may be just an impression—that writers today—particularly—but not entirely—the younger scribes—have an inordinate fondness—perhaps even a mania—for using dashes.
While I'm as fond as anyone of the classic em dash, it can get a little distracting. That's the point—or rather, it is not the point, since the point is, as everyone knows (or maybe I should say—as everyone knows), is the period, full stop, basic end punctuation for all declarative, non-exclamatory statements. But I digress. (Digression also is a function of the dash, as well as of its brother the parenthesis.)
A dash is what you use when you want to literally make a break for it—that is, make a break in your literature. See what happened there? I broke up the sentence like a brittle twig and bent it in the middle to call attention to my little bit of wordplay. Ha! Made you look! That's what the dash does; more than any other form of punctuation, it makes the reader do a double take. "What's this?" the reader's subconscious says. "This looks like an aside. I'd better pay attention to it."
But when the dash is used too often, that break-up effect starts to interfere with the narrative flow and impede the reader's absorption of information. It can also be a crutch that unskilled writers grab whenever there's a random fact or definition they don't know how to fit into their structure, ending up with sentences like these:
Instances of road rage—numbering 87,000 in 2011 alone on U.S. streets—are on the increase.
"I've known about Mr. Varman's work on the project"—which promises to transform the field of particle physics—"for 20 years," said Lily Xavier, president of the U.S. Amateur Science Association.
If you have to break into the middle of a statement or a direct quote in a way that opens up a wide gap between the start of the thought and its conclusion, it may be an indication that you need to separate that extraneous information into its own sentence, and possibly even find a way to restate that thought.
Dashes have also taken over the roles of other forms of internal punctuation. They have supplanted the comma:
The talk-show host led the magician to the stage—where a large box and a set of props already awaited her.
They have usurped the semicolon:
An investigation found irregularities in Epgo Inc.'s finances—afterward, the company fired 10 employees in the accounting branch.
And they have committed regicide against the colon:
Five of the 200 companies that entered were awarded the Marbleton Efficiency Prize—Varma Insulators, Kryptik Lock Co., Mabronics LLC, Eames & Son HVAC, and Yurupyurdown Elevator Co.
The best way to guard against overuse of dashes is to think of the wealth of punctuation options that are available and try to employ the one that best suits the function. Sometimes you don't want a generic form of strong separation; you want a semicolon, which indicates the beginning of a related independent clause. Or, if you want a form of punctuation that will interrupt but not hijack the flow of thought, you might choose a comma. If you have a list or if you want to indicate an idea that is a direct consequence of the preceding idea, you might use a colon: It provides the needed separation, but it announces its purpose clearly because of its limited role.
An ardent fan of dashes might object that Emily Dickinson used dashes liberally in her work and was successful in communicating her ideas. It's true that she didn't so much sprinkle dashes into her poetry as pour them with a ladle, but look at how they are employed:
From Blank to Blank—
A Threadless Way
I pushed Mechanic feet—
To stop—or perish—or advance—
If end I gained
It stands beyond
I shut my eyes—and groped as well
'Twas lighter—to be Blind—
Dickinson used dashes to create a sense of space, separate out thoughts, and slow down her readers, forcing them to consider each idea separately. She knew full well the shock value of dashes, the way they arrest the eye's progress across the page, and she used them because they were what best suited her purpose, not because she was unaware of her other options.
For the prose writer, however, a full toolbox of commas, semicolons, colons, parentheses, and ellipses will help clarify meanings by serving a more specific and usually less intrusive function than the all-purpose em dash.