I believe a cover letter for your resume is important. While the resume describes your work experience, a cover letter is able to relate the benefits and capabilities that you will offer the organization in order for it to achieve greater success. This shows that you have taken the time to research the company. If two people are being considered for the job, a well-written cover letter may elevate one candidate over the other. Also, writing a cover specifically addressed to someone within the organization shows that you have taken the time to identify your target. Understanding the companies goals enable you to write a more personalized, detail-oriented cover letter about the organization. Sending out a mass cover letter most likely only contains information about you and cannot be tailored directly to a specific organization. Adding a hand-written postscript is also a nice touch. It will easily stand out against the other printed material. A testimonial might also be a nice touch to reinforce the quality of your work. This would reinforce the positive statements written by you in your cover letter.
I think all resumes should be accompanied by a cover letter. Even if they are never read, they show the employer that you have taken the time to research and evaluate the company.