> MOST of the time I think the cover letter either gets a brief glance or doesn’t get read at all.

That might very well be the case for a big company that receives a ton of applications but the numbers game works both ways: as the person looking to hire somebody, I find it well worth the time to weed out people based on poor spelling, grammar, etc. It only takes a few seconds to spot the bad ones, and it ensures I don't have to waste my time with somebody who has poor communication skills to begin with.