I often ask for cover letters. Sure, the extra few minutes it takes to write one hopefully filters out a few people who can't be bothered, and it's also a way to test people's English. But actually the main reason is I genuinely want to get a quick notion of who this person is and why they are applying for this job. No hidden agenda. Depending on the situation, I'd be perfectly fine with "I desperately need a job and so am applying to all [job title] positions I can find. I know nothing about your company, but had a quick look at the website and it looks like it could be interesting." So it's a shame that the majority are so formulaic and insincere as to be useless.