I was just about to set up an EC2 micro with node and mongo this weekend.
I also get the opportunity to build a server, deploy in an environment that I can scale in "one day"
I think there are a lot of valuable skills to be gained by doing this for free and for fun right now, so that I can do it for a startup later on.
If I don't know how to build an EC2 system, I can't be confident in my ability to sell that skill later on.
Especially with node.js that simply doesn't make sense.
If you're running it with users on it which would actually need CPU, memory or disk, duh I would run it on EC2.