I have never once seen a real hiring manager look at these, most good candidates aren't going to be writing ones specific to your company. I'm sure its nice to be 37S.
When hiring staff members that need to be able to write documents as part of the job and explain things clearly, the cover letter can make a significant difference. Most good candidates do write ones specific to your company. A generic cover letter is an extension of the resume (at best) and often an indicator of a lack of interest (at worst).
I have no idea why you think most good candidates won't be "writing specific cover letters for your company". Top talent doesn't shotgun resumes across 100 companies; they pick places they think they'll like, have a reason for doing it, and do everything they can to get the best possible offer.
I spend a good 30-45 minutes building a list of questions based entirely on the contents of their resume. I couldn't imagine just looking at a resume and then interviewing a person off the cuff. Seems a little rude.
If they are generic form letters that have been obviously sent to everyone without any modification, then I probably won't even talk to the person applying.
If they aren't interested in what we're hiring for, it makes me a lot less interested than what they have to offer.
But, my experience on the other side of the table is that I've never actually gotten a cover letter passed on from HR. Or, if HR gave them to the hiring managers, they didn't pass them on to the engineers reviewing resumes & doing interviews.
Maybe there's a strategy here:
Cover letter: I love frat-houses and beer! I want to bring my spiky hair to work for you!
Resume: When I'm not submitting patches to the RoR team I'm writing Scala code for my EC2 cluster.
It's a shame the economy is so bad. It would be fun to do some exploratory job hunting to see if this strategy would work.