[dangerlibrary]

The best argument for a cover letter over a resume is that relatively few people tailor their resume to each application. As a result, a resume is the job-application equivalent of a mass-mailer. Cover letters (when written correctly and not templated) convey a certain amount of interest in THIS job, and show that the candidate is serious about the position.

[Kalium]

I think that's only significant if you believe a candidate's display of interest correlates significantly with their ability to perform on the job. I am not convinced that that is a significant correlation or that it should be aggressively selected for.

[dangerlibrary]

It definitely correlates with interviewing them being a complete waste of time because they barely read the job description and don't actually want to work for you.

[Kalium]

Then I suppose it's a matter of what your local hiring market looks like. If there's a lot of unattached talent floating around, by all means, filter aggressively for those who want it the most.