Maybe my industry (consulting) is different but yes, absolutely yes, this is a tactic with nice ROI. I can't think of a single good reason not to leverage your existing contacts.

References and testimonials, LinkedIn, Zerply, Github, et al? Maybe I'm too suspicious; if after an initial interview I got a stream of emails from an applicant's buddies/colleagues that I didn't ask for, I'd wonder why he felt he needed to coerce them to do so. I also think it's a bit dangerous to latch someone's technical skills to their social aptitude. Everyone has friends, not everyone thinks it is professional to sweet-talk a founder on your friend's behalf. I'd love to hear what others think, though.

I think it comes down to discretion. I would never recommend flooding an interviewer with internal email, but one helpful nudge from a respected colleague who vouches for you won't hurt.

I can think of a good reason: it's a negative sum game, because willingness to proxy-spam a potential employer has no positive correlation with the ability to do the job; if people start doing this, everyone will be worse off. So if you're an employer and you don't want to end up on the receiving end of this kind of spam every time you hire and under pressure to send it every time one of your friends is looking for a job, you should reject anyone who tries this tactic.

According to the article, the person in question was applying for a technical recruiter role. Given that the biggest problem tech recruiters at startups face is arguably getting top-level engineering talent to give them the time of day, I'd say that a stunt involving getting a lot of talented engineers to talk about how much they like and respect you is completely justified and appropriate.

Perhaps where I'm getting tangled is in using the term "friends" instead of past recruits or former hiring managers. Regardless, I still think it's a poor practice, given that -- like rwallace said -- if everyone did it, it would just create even more emails that the OP needed to add to their laundry list.