The most valuable of all talents is that of never using two words when one will do.
Cover letters are extremely limited in value, even when properly used.
First, most people assume that the cover letter is actually read before the resume. In my review of more than 25,000 resumes, I have probably read only 5,000 cover letters. It is actually rather amusing to watch a hiring manager reading a newly arrived resume. The cover letter and resume are pulled from the envelope, the cover letter is immediately placed behind the resume, and the resume is scanned first, then read. And you know there is interest if they finally make their way back to the cover letter.