It's hard to tell, I can only guess:
Why it's bad
First of all, further fragmentation of the server-side development community. Aren't there enough ways to write web-applications as it stands? Multiple languages to choose from, multiple frameworks to use once you've chosen a language. Do we need another language & platform for server-side development? What's so bad with having two tools, each suitable for its domain?
Why it's good
Just as the rise in popularity of JS in the browser brought us faster engines that make the internet experience more pleasant, I think that examining new approaches is generally healthy. There's a lot of excitement around this issue lately - excitement that brings talented developers in, places them in the zone for productive hacking, and overall provides a fresh perspective on things.
This approach isn't new. Twisted is a venerable and powerful event I/O library for Python, for example. However, when you code with Twisted you have to be careful to use only very specific Python modules - use one that is blocking and you've ruined the whole model. Node.js, on the other hand, comes with its own standard library that was designed from the ground-up to be 100% non-blocking.
On yet another hand, making one process run as fast as possible is a lofty goal, but what about true parallel programming? Event loops such as the ones offered by Node.js don't scale to multi-CPU machines, so I wonder how this situation will be handled. My bet would be on relatively independent processes communicating via message-passing and talking to mutual data stores via TCP.
These are exciting times. The vibrancy reminds me of the Rails hype of a few years ago. As I've mentioned, from my point of view this situation has its pros and cons, but the pros outweigh the cons. Being locked in narrow directions hasn't been historically fruitful. Mutual enrichment between communities and arms races are a much more viable way to progress.
|||Which, by the way, was specifically designed by Google to serve as an embeddable engine.|