> My resume spells out, in thorough detail, why someone would consider hiring me.

I always thought that the resume should explain why someone would want to hire me ("look, I'm smart, know some stuff, and did some cool things!"), while the cover letter explains why you in particular want to hire me.

I had very good look writing cover letters where I pulled apart the job description and explained how my experience (described on the attached resume) fit with each of the requirements.