No surprise that I agree with you completely. :) Cover letters tell me so much about a candidate, how well they communicate for one. They also help me prioritize. Write a great letter that points out how you match the job well and I'll eagerly open your word doc, anticipating a great resume.
I'm working on a search now where communication skills are a big part of the job and am amazed at how few people send any kind of a detailed letter. The ones that do stand out in a sea of resumes. Interestingly, there seems to be a correlation between quality of cover letter and quality and appropriateness of candidate resume. The resumes that have no letter are the least relevant in most cases and I suspect they're just firing off the word docs to any job that catches their eye (or that has an intriguing title...doubt they even read the description in many cases.)
Only thing I'd advise people is to not send the cover letter as an attachment, unless you have to. Just paste into the email and attach the resume. Otherwise, if I get an email with two attachments, I usually jump right to the resume, which isn't always a good thing.
when I was making placements and all the recruiters in my office were making placements, I never read cover letters. we were placing engineers and I never found a cover letter necessary. Most engineers we dealt with had one page resumes and that was more than enough in my opinion to know if I thought the candidate had the skills to do the job I was recruiting on and if so, I'd make a call.
If a cover letter has additional information in it that is not included in the resume and shouldn't be i find them vital for presenting a candidate. Sometimes i will include a cover letter when i submit a resume ,if it's a good one, along with my interview notes on the candidate. With IT candidates it helps if they include some detail about technology. Accountants, not so much. Engineers, i agree with JD, never had a particularly good cover letter from an engineer. With a lot of the scientists and reasearch PHD's that we work cover letters work.
If the cover letter has some indication of salary history and reasons for changes i will spend a lot of time reading them before i call a candidate. If the candidate includes something like the fact that his wife's family or his live in the geographic area where the position is located i will call that candidate before i do others with everything else being equal.
I will also read the resume first if the cover is in a seperate doc but will always read a cover letter unless as discussed it is one of those four liners that says nothing. I don't think the summary at the top of the resume takes the place of a good cover letter. I like a cover letter that is more personal in nature.