A List Of Things Thrown Five Minutes Ago

ALOTT5MA FRIDAY GRAMMAR RODEO: As several of you pointed out to me during the week, Slate seems to be on a grammar kick of late and this week takes on the 'em dash':

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.)

See, related, this NYT editor's lament about the overuse of the 'em dash'.  Meanwhile, The Economist's style guide suggests:

You can use dashes in pairs for parenthesis, but not more than one pair per sentence, ideally not more than one pair per paragraph.

Use a dash to introduce an explanation, amplification, paraphrase, particularisation or correction of what immediately precedes it. Use it to gather up the subject of a long sentence. Use it to introduce a paradoxical or whimsical ending to a sentence. Do not use it as a punctuation maid-of-all-work.

And the Chicago Manual of Style outlines the differences between the hyphen (-), en dash (–), and em dash (—).