Some people don't though, and their referrals are much more valuable than dozens of additional applications.

Much like many other decisions you're making in the job market, it's a polarizing choice that increases your overall chances when the alienated class of people isn't too large. If 90% of people ignore those emails but your chance of getting a first-round interview goes up 5x compared to a cold application when the remaining 10% respond, 2+ emails are easier to send than 1 application, especially when you've done the legwork to make your application any good.

I haven't used techniques like these specifically yet, but as somebody who nearly always eventually gets the job once I've had a first-round interview, I wouldn't be opposed to seeking out the hiring manager and contacting them directly to decrease the resumé false rejection rate.