Having sat through years and years of presentations and documentation reviews, I'd say most developers, even highly educated ones, can't write decent prose so a

Having sat through years and years of presentations and documentation reviews, I'd say most developers, even highly educated ones, can't write decent prose so a cover letter displaying that would hurt them. Even if they have someone else polish their cover letter, that's still them presenting themselves as having a level of writing talent that they don't actually have.