Here's what super-recruiter, Anthony J Meaney, says about cover letters:
1. Most recruiters don't read cover letters.
2. Most cover letters don't say much.
3. There is no standard format so they are hard to navigate without reading them word for word.
4. A resume with it's well-defined statements of education, companies, dates and titles offers a faster scan.
5. Don't write a cover letter. Put a note in the body of your email saying: Please find my resume attached. I believe I have the necessary skills to do the job, I will follow up with you in a week to answer any questions you might have.
Animal Wisdom: I believe that a cover letter is a good place to list, schematically, where your experience matches the requirements of the job. No over-selling is required. This helps a recruiter who doesn't understand what you do, pick you out as a potential candidate from a mass of other resumes.