The problem with this is that Node.js isn't included on Mac OS.
I've been looking at whether it would be possible, but I think the only way would be to distribute the Node binary with the workflow.
The list of supported languages are all installed by default on Mac OS, but Node needs to be installed manually. You could just assume that it is installed and provide instructions to users that they must install it, but this isn't a great user experience and many people won't be able to do it.
To allow Node scripts in Alfred would require distributing Node.js which is 10MB. Considering most workflows seem to be < 100KB, it seems unreasonable to include it with one, especially if multiple workflows are all going to require it. Alfred 2.0 is also 4.6MB (uncompressed) so I doubt Andrew wants to include it by default.
It is possible to write a system to run JavaScript plugins using JavaScriptCore which is included in Mac OS, I'm actually writing one myself for another project, but the downside of this solution is that it is not a Node environment. You don't get NPM, no Node modules will work, and all the file system/http/etc APIs need to be written from scratch. This is a huge amount of work in itself and as Bash, Python, Ruby and PHP are available there is really very little to be gained from this work.
Python and Ruby are good languages with great communities, lots of documentation, support, and libraries that have already been written. I highly recommend them as an alternative to JavaScript.