One major advantage I can see is if you're a full stack dev. You don't have a switch languages when you're building the backend and the front end. Some people may not see this as an advantage, but personally it takes some time for my brain to switch to javascript for the front end when I've been using either java, python, or ruby for the backend.
The right tool depends on the requirements. If there's no special requirements then I don't see why javascript can't be a substitute for ruby, python, php, or perl on the backend.
> you would use something like couchdb instead of a relational db
I'm not sure this is a good analogy since javascript (as well as the backend languages that it seeks to substitute for) is more of a general purpose tool, and not a super specialized datastore.