100% this. When I was interviewing I almost never got the cover letters from HR. When I did glance over them every time it was obviously a form cover letter that they spent 15-30 minutes customizing for our company.

If you have something incredibly unique to tell the company like "I want to work for this pharmaceutical company because your newest chemotherapy drug saved my wife's life" I'd sneak it into the resume.

Proving to the interviewer that you can write three sentences of generic fluff based on a cursory glance of my company's website doesn't tell me anything about your abilities, just how much you value your time.