I find cover letters to be important. They provide a narrative which allows me to get a better feeling for the applicant. The interview is far more important, but not only do cover letters provide background, they also tell me what kind of a writer the applicant is. Being able to write well and communicate clearly is really important for just about every industry. Technical aptitude is great, but if you can't communicate with the end user and management, you're not going to be as effective as you could be.
I wouldn't just reject an application without a cover letter outright, but if they can't be bothered to write a couple of paragraphs, I have to wonder how much they really want this job.
Oh, pro tip - if you're going to reuse a cover letter, make sure you change the name of the company to which you're applying. I've seen several letters saying "I'd love the chance to discuss working at Company X!" That's great, I'll forward it over to them.
