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.