This has come up multiple times on ask metafilter. You could search the archives—I'm too busy thinking Big Thoughts about Kant to do so on your behalf. (Right now I'm thinking Big Thoughts. I'm even having Sizeable Cognitions.)

My thoughts are that it's good that there's makework of a sort for the type of person who's likely to want to become an HR goon, but it's gone a little too far.

I can tell you that when my boss receives a really bad one, she often runs around the office reading them aloud for our amusement. It's amazing how many people send them out with huge mistakes in them. Like, oh, spelling the addressee's name wrong. So don't do that. Really I think cover letters are just a filtering tool. If you get one that looks like it was written by total moron, then you don't call them. But you are not a total moron, so I'd dare say you should relax and just make it straightforward. Why are you interested in this job, and how does your experience make you a an excellent candidate.

I think it varies by the person and position. To some people (and for some jobs) every resume starts to look the same. The cover letter can bring more of a sense of the person. And for journalism, it's the first sample of your writing that they'll ever see.

With that said, in what little interviewing experience I have the cover letter has almost always hurt the candidate. But I think that's because people are mostly idiots who don't know how to write.

I got a resume with headers in Comic Sans today. I shit you not.

(Sorry, not quite on topic but it's just SO WRONG I had to tell the Internets.)

I had assumed that Comic Sans was, at this point, a no-need-for-discussion issue. At least among moderately web-savvy, snarky friends. Then, at a much-respected friend's house, I found a lengthy political missive, indicting the current Administration for all sorts of evildoing...written in fucking Comic Sans.

I wept. Inside, at least.

I have hired plenty of people to do marketing and web and PR jobs, and I always read the cover letters as most of the positions reporting to me are writing-intensive. I'd definitely include one. Like someone else says, it gives a sense of your personality and interests beyond what's listed on your resume. If you are looking for work in a different city than you currently reside, or if you will need to be part-time or have some other consideration, the cover letter is the perfect place to explain your availability and requirements.

Feel free to email me if you have other questions. I've seen many a resume and cover letter in my lifetime.

Sorry, I have no advice; I'm just happy that the phrase "stabbed myself in the eye" is finally spreading.