I wondered this too, so I went out and implemented a large project in Node (Haraka - an SMTP Server). It turned out to be such a success that Craigslist dumped

I wondered this too, so I went out and implemented a large project in Node (Haraka - an SMTP Server). It turned out to be such a success that Craigslist dumped Postfix for it (an SMTP server written in C), and managed to get rid of more than half of their servers due to Haraka being faster. We just added another example company doing the same - bounce.io - they are seeing similar gains. Yes Node has problems like any language does, but the gains are real.