It seems pretty high payoff for people who might otherwise not get a phone screen. When I was tasked with shuffling through resumes and rating them (and seeing other engineers do the same), less than 5% of applicants had cover letters, and the presence of a cover letter almost guaranteed a phone screen. This included really weird cover letters that dedicate a paragraph or two to food or pets or something, but not cover letters that are obviously sent out to many companies with at most a few words changed.
