Oh yeah. I learned this lesson very quickly. Editors don't care about your cover letters. (nb. I speak only for editors who are me.)
Seriously. I never agonize over my cover letters when I send out stories, I keep 'em nice and generic, but I always mention my sale(s, hopefully) and my VP attendance, thinking they'll count for something. Now I feel like all that is vanity.
On the guidelines page I say that if you want, you can mention stuff like publication credits and your name. Some people are taking this way too far! I'm getting multi-page cover letters. I don't need this information. I'm buying a story, which you've thoughtfully attached to the message. I'm not hiring you to run Accounts Payable.
But you could shrink the cover letter down and down to the length of my typical cover letter ("Hi, here's a story; one time I sold a different story; well, gotta go!") and it would still have more information than I need. Because stuff in the cover letter is stuff that's external to the story. (Hopefully.) There will be plenty of time for that kind of thing after I buy your story.
One thing I've found I do like in a cover letter is when you show that you've read the submission guidelines. I mean, most of us are used to sending form cover letters in with our stories and getting form rejection letters back. It's nice to be reminded that there are real people on both ends of the process. But on the whole it seems a semi-archaic practice, like calling cards. Maybe I'd feel differently if I ran a regular magazine and had a chance to build up long-term relationships with authors.