Remember, it takes recruiters an average of 5 seconds to read over your resume. If they take the time to read your cover letter, think of the time they will need to read everyone else's cover letter. In fact, this will double their work. Despite all the hype, cover letters are less effective than resumes. Though Human Resource personnel may receive your application materials, chances are that person will read your resume over your cover letter.
Resumes are easier to skim over, and provide information in a convenient format.
On the other hand, cover letters can cover a variety of topics from why you're applying to your whole educational history. Remember, the Human Resource department is only the first line of contact before you get to the real decision makers. While I was working at one of the top entertainment companies, I had a friend who worked in the Human Resources department. Part of her job was to separate the resumes from the cover letters, separating them into two piles. Recruiters would read the resumes and toss the cover letters in the trash.