The cover letter is a controversial thing. I hesitate to communicate my thoughts on this topic because I am sure that many people labor over the cover letter and spend an enormous amount of time on it.
First, what should the cover letter include:
The title of the position that you are applying for
List how or where you learned about the position (if there is a job code include it)
Parallel your background/experience and how you can be of value to the future employer based on what "they" are looking for
Your availability to talk to the hiring authority and the best way to reach you
Your contact information
Now, do you need a cover letter?
Answer: it depends.
No if,
Someone who is already connected to the company/position is putting forth your information
You are applying for a specific position
You have the direct contact name for the person overseeing the hiring process
Yes if,
You are not applying for a specific position
You are seeking an informational interview
Have a unique aspect of your resume that needs to be explained
Are applying for a position where your skills are not current in the specific field (the cover letter will allow you to explain why you feel you are a fit)
You are applying for a marketing, communications, sales position- often your written style will be a assessed as a part of the interview process!
They are asking for one
My experience with cover letters:
When I was hiring (as a Manager of HR) I typically went straight to the resume to get to the meat of what the person could do/bring to the company/position. If the resume had positive attributes (longevity in positions, growth/advancement, relevant experience) but, they were not a direct fit, I would reference the cover letter to get a sense for why they felt that they were a good fit.
Cover letters can be valuable for showing your ability to communicate via writing, which is critical in many professional level positions. It can also "sell" your background differently than a resume.
When the top 5% bubble to the top, the cover letter may become a deciding factor relative to who a company brings in for an interview. Especially if the position is in marketing, sales or communications!
My thoughts on the cover letter:
Given a saturated market you may be laboring over a cover letter that is never read! Recruiters/hiring managers are so busy trying to get through the resumes that reviewing every cover letter can become very time consuming. Keep in mind that cover letters are often poorly written and are done in a lot of different formats which requires more time for the "reviewer"!
According to data released in the Job Openings and Labor Turnover Survey report published by the U.S. Bureau of Labor Statistics in '09; there were 5.4 job hunters for every advertised opening in April. This was up from 4.8 in March and up dramatically from 1.7 in December 2007, when the recession began. Note: As of April '14, the BLS had not published updated information.