I heard recently from someone who took the time to target both her resume and her cover letter. She wanted to let me know how well it had worked out for her. Both specifically linked her skills to the criteria listed in the job posting. She got called for an interview shortly after submitting her materials.
She was offered the job shortly after the interview. The hiring manager commented that "I wanted to hire you as soon as I saw your resume." This is just one example of why taking the time to target your resumes and letters is beneficial.
When you do some of the work for the employer - showing them why you are qualified for the job - you are making the hiring process simpler all the way around. You're also going to have a better shot at getting interviews and getting hired. So, do put the effort into created targeted job application materials - it really is worth the extra work.