Do Cover Letters Help Get Interviews, Or Are They A Waste Of Time?

At the Secrets & Strategies career management group on LinkedIn this week, they were talking about some jobs guru who claimed cover letters are a waste of time and how you'll get more interviews if you stop using them.

Another authority was cited as saying that hiring managers are peeved when they don't receive cover letters with resumes. Most people expressed confusion and exasperation, so I thought an answer to the dilemma would be useful.