The Role of a Cover Letter vs. a Resume
In some cases, this is true. Some employers give a great deal of credence to the cover letter, especially if you are applying to a writing-intensive position. That's because a cover letter serves as a better indicator of writing ability than a resume, which is usually comprised of short, choppy sentences, a.k.a. "resume speak" or as we sometimes call it "caveman speak." In addition, cover letters often provide information, such as your reason for seeking a new position, which cannot be found on a resume. They can also tie together the transferable skills you derived from your career as a whole, which may not be obvious simply by viewing the resume.
In many cases, however, this notion is false. Many employers will look at your resume first and will only read your cover letter if they deem your resume worthy. Some employers never read cover letters at all. In some large companies, the cover letter is tossed, and only the resume is passed on to the hiring committee. That is because, as discussed previously, there are typically a few key things that employers look for before deciding whether to spend any time reviewing your materials in depth.
If you have absolutely no experience in a given area and the employer is looking for 5-7 years, your application will be dismissed once that is discovered. If you went to a particular school or received an unusual degree and the employer has already decided not to consider anyone with that educational background, you will be immediately dismissed as well.
That said, just as you should always wear clean socks "in case someone sees them," you should always have an excellent cover letter "in case someone reads it." Some employers like reading cover letters. An employer might happen to glance at your letter while drinking his/her morning coffee and read it closely just because it's there. In some situations, like this, your cover letter will have more of an impact than your resume.
So the bottom line is this: you should draft your resume in such a way that it stands on its own in the event that your cover letter is never read. In other words, if you feel that something is vital to your candidacy, it should either be mentioned in both documents or in the resume only. You should draft your cover letter as it if will be read, but don't rely on it to convey too much vital information that doesn't appear on your resume.