Many people are convinced cover letters are out, but whether you need a cover letter or not depends on the job you're applying to. If you're lucky and have a contact who is willing to pass along your résumé, you probably don't need one. If a job description asks for a letter or states that it's voluntary, you'd be smart to write one.
Even though it's almost guaranteed an employer will not scrutinize your cover letter like a teacher would, it's still important. If your résumé has attracted the attention of human resources or a hiring manager, they may read your letter to get to know more about you. If they don't, the mere fact you've taken the time to write one is important. It's like the thank-you note you should always send following an interview. It shows respect and motivation. If you have qualifications that are equivalent to those of another candidate, and she did not take the time to write a cover letter, in whom do you think the employer will be more interested? That's right: you.