I will play the devil's advocate and disagree.
I have never used a cover letter and I have had good success when searching for new positions. I have a second interview Monday for a position that would make my kids' college funds very happy. I have 3 daughters and if I can keep at least two of them from dancing on a pole for a living, my life will have been a success!
It depends on your experience. It depends on the position. When I have interviewed potential hires in the past, I was never too concerned about seeing a cover letter. The references, cover letter, and extra information often times get held on to by HR or the recruiters anyway. The hiring managers often times just want the resume.
A cover letter is a personal preference in my opinion, not a game changer.
I don't think it's a negative thing to do and unless you write it incredibly poorly or include inappropriate information, then it can't hurt.
HOW IMPORTANT IS TO HAVE A COVER LETTER WITH YOUR RESUME?
I NEVER MAKE IT AND MAY BE THAT IS WHY I AM STRUGGLING TO GET A JOB EVEN THOUGH HAVING I GOT WHAT IT TAKES ME TO SECURE A GOOD POSITION.
If you're a hiring manager and you've got a stack of one hundred resumes on your desk, are you really going to take the time to thoroughly read a cover letter? No. You're going to skip to the resume, skim it, and then decide whether it goes into the "yes", "no", or "maybe" pile.
Instead of writing a cover letter, spend your time reading about the intricacies of EIGRP or brew me a coffee. No sugar, please.