That's not really true...
Node.JS is amazing at doing asynchronous I/O without consuming huge amounts of CPU. It's also very, very quick and easy to develop in.
Thus if you have an app that expects to consist largely of asynchronous I/O, it's well worth it to use Node.JS even if the per hour cost to build it is higher. In the long run you will save money building (likely many fewer hours to build, even if higher per hour cost), and operationally you will save money on hardware expense.
That being said, if you are building with any tool purely for the sake of using that particular tool, you're doing it wrong. Academics in particular are guilty of doing that, and often.