Where would writers be without the helpful em-dash?†
A dash, the style manuals helpfully state, indicates a break in the sentence, or (as part of a pair) encloses a parenthetical statement. In practice, this makes the dash an informal replacement for several other pieces of punctuation: parentheses, the colon, the semicolon, even ellipses.
It’s the protean nature of the dash that presents the greatest hazard. Being so useful in a variety of different situations, it’s vulnerable to overuse. A writer who isn’t careful can end up with a page full of dash-filled sentences, which lends a sort of panting urgency to the prose that usually isn’t wanted. The best general advice I ever read on the subject was a stricture I encountered in a media fanzine back in the pre-internet era, and it ran something like this: “If you use more than two dashes in one paragraph, you aren’t allowed to use any at all in the next. So there.”‡
There’s a higher-level version of the same problem, involving semicolons. There are some writers — I plead guilty here — who like semicolons entirely too much. If I’m not careful, I can find myself committing a three-sentence paragraph where all three sentences feature independent clauses joined together by semicolons. At that point, I have to force myself to take an axe to at least two of those sentences and break them back up into their component clauses.
Sometimes, though, I cheat, and replace one of the semicolons with a dash instead.
†There’s also a shorter version, the en-dash, but its uses are much more restricted and frankly, if you use a hyphen nobody’s going to call you on it. Well, maybe the typesetter, but unless you’re being your own desktop publisher, you aren’t likely to ever meet him or her.
‡I believe the zine reviewer in question was noted Star Trek fan Paula Block, but at this remove I can’t be sure. Whoever it was, I owe her for the words of wisdom.