The main advantage of Node is that you're already using Javascript on the client side, and this lets you share code. If you've got business logic that you want to use on both client and server, keeping them in synch between two different languages is a nightmare. It's pretty much the epitome of Repeating Yourself.

That said, it's kind of a narrow domain, where your server-side logic is simple enough that you can successfully write it in Node, but complex enough that there's code you can't afford to keep synched.