Interesting to know that there are exceptions to the rule, but on the whole the cover letter isn't something I'd optimize for. I just don't think it's used that much.
The cover letter is your chance to explain Why you want to apply and Why you'd be a good fit.
The resume explains Who you are (professionally).
The Why is just as (and sometimes more) important than the Who.
The gap in your chances between "I know someone in the company" and "I don't know someone in the company" is so huge that I can't/couldn't imagine a cover letter making a big difference if you don't already know somebody. So my recommendations have normally been around networking or canvasing friends-of-friends to know somebody and then go that route.
That said, I have to admit it's been a long time since I've had to do a cold round of applications so I totally accept I've underestimated.
A Cover Letter / CV / "Inside Source" isn't (usually) going to get you the job. It's just going to get your foot in the door.