Back before NodeJS was cool and was still just a struggling new project, I listened to a podcast interview with the original author. The original developer wanted to create something that was equivalent to Twisted (a Python framework) but was able to use the V8 JIT compiler from Chrome. He thought that Twisted was fantastic and was a real fan of it, but Python didn't have a working JIT (Pypy didn't work with Twisted at the time) and he thought he could build something faster.
The justifications along the lines of "use the same language for front end and back end" came a lot later on and seem to be very much after the fact. The original reasons were that Twisted was really cool and V8 was really cool and sticking the two together somehow would be even more cooler. This sort of enthusiasm is how stuff that actually works tends to get built.
I've compared NodeJS to Python (CPython, not Pypy, so no JIT) with Twisted, and found that in terms of performance, which was faster depended on what you were doing. The NodeJS event loop was faster (but less flexible) but I was doing a lot of JSON parsing and the Python JSON library was faster. In addition, things which required a load of Javascript code may be a single instruction in Python, which negates some of the theoretical advantages of having a JIT. Overall, performance depended on exactly what you were doing, so you can't draw across the board conclusions.
In terms of how much work it took to write each version though, the NodeJS version took a lot more lines of code than the Python/Twisted version. A lot of things which are a one-liner in Python takes multiple lines of code in Javascript. If you're looking to save development time and costs, that's a very big consideration.
As for being able to write the server and the web scripting in the same language, I don't really see much advantage to it. I've never had any problems using Javascript for the front end and Python for the back end. But then again, I've been programming for longer than either language has been around and have learned a number of languages. If you've got someone who says that they only know one language and it's too hard for them to learn another, then I don't think that using NodeJS (or Io.js) is going to solve your real problems.
Biting the hand that feeds IT
