I've seen this addressed quite a lot recently in blogs and career forums, with people claiming cover letters are useless and never read. Clients frequesntly ask if they need to write one at all. The simple answer, in my opinion, is simply YES.
First of all, a cover letter is a much more personal document than a résumé. It not only shows off your writing style, but it is a chance for you to mention details or anecdotes that simply don't fit within a formal résumé structure. Don't lose this opportunity to further sell yourself to an employer by leaving the cover letter off.
The cover letter is also your chance to demonstrate what you know about your potential employer, and how you will fit into their organization. You are using your résumé to show your professional experiences and how you contributed to the success of past employers. In your cover letter you take this further, and say "look at what I have already accomplished - I can do all this and more for you."
Critics argue that letters are long and dull, and quote top recruiters who say they don't read them at all, to make their point that a cover letter is a waste of your time. In truth, your cover letter may not be read. There's nothing you can do about it. Or, it might be. You just don't know. Do you really want to be the one applicant in twenty who didn't write one? Do you want to allow that to lose you the chance at an interview?
When screening candidates I often skip right to the résumé on first glance, taking time to read the cover letter only when I have seen that the candidate meets minimum qualifications. But I do look for it. I've received résumés without cover letters before, and my assumption has always been (right or wrong) that the person couldn't bother take the time to learn enough about the job and organization and write one. No one wants to hire that person.
What are your thoughts? Do you include a cover letter? Hiring managers: do you read them?