Comparing JS libraries with web server frameworks is pretty apples-to-oranges, first of all. But MVC frameworks pretty much have to be "everything". Can you use a desktop UI library without going with it completely?
This is more of an apt comparison for web front ends. The choice of Backbone (or, in my case something that provides a little more value like Flight) vs. Ember/Angular is like choosing to build a CLI app vs. a full-on desktop app.
There are perfectly valid reasons for choosing either and in the right context, the right tool is worth its weight in gold.