If you overuse the dash, you will exhaust your readers. Some writers use dashes to cram three or four ideas into one sentence (The work of David Eggers—that most beloved writer of thirty-somethings—while highly entertaining, and certainly high-spirited—frantic, some would say—still sells remarkably well); some writers use them to indulge their own discursive thoughts; some writers use them because they are lazy and it is easier to compose one sprawling, dash-filled, dubiously organized sentence than it is to compose three tidy, logical sentences. If you fill your writing with dashes, you force your readers to stop and start, to back up and then plunge back into the sentence.