This is one of those questions for which no “right” answer really exists because each recruiter or employer has their own preference.
I recently saw a panel discussion by recruiters from very large companies, including Microsoft and Starbucks, and each recruiter had a different response to a question about cover letters being useful.
Some recruiters said, “Don’t bother with a cover letter. We don’t read them – they get immediately thrown away. We go straight to the resume to see if it contains what we need.”
Other recruiters said, “The cover letter is very important. We won’t even look at the resume without a good cover letter that provides us with a reason to look at the resume.”