You might send out 20 resumes. As a hiring manager - especially when using a "no filter" approach like posting on here (keep in mind that IME people posting jobs on Hacker News are more likely doing "guerilla hiring" and circumvent their HR departments) - I easily get 100+ resumes, and I don't have days to spend trawling through them. I think it was rands of randsinrepose who mentioned before that the average time an HR person or a manager gets to spend on figuring out if a deeper dive is worth it is about 30s.
So I have to filter them by something. If you as an applicant - who would like at least an interview with me -can't even put in the effort to write in a few sentences that tell me why I should look at your resume, I somehow have to assume that you'll put in similar effort after I hire you. That's not a good first impression and IMHO it counts when you're dealing with a hiring manager as opposed to an HR department.
Keep in mind I'm not asking you to write a cover novel. Put yourself in my shoes and tell me which of the resumes you'd look at if you have only limited time:
1. The one that says
"My resume attached", or fewer words to that effect
2. The one that says
"Hey, I'm applying for your job opening because I love writing web apps in Ruby on Rails with a ClojureScript front end".