Backbone has always been intended to give you the smallest possible set of functionality that you reach for every time you work on a JS-powered web app. Model and Views and events to bind them together, collections with powerful functions for filtering and aggregating data, and shareable URLs.
There are lots of other directions and other needs that JS apps may have, but unless it's something that you're going to use pretty much every time, it doesn't belong in Backbone itself -- it belongs in an extension, or better yet, another standalone library. I'm talking about things like write-through localstorage caching, pushing data over websockets, TreeViews, query syntax for complex JSON, special cross-domain communication, specific HTML-generating strategies, or "mobile" widgets that mimic iOS controls.
Fact is, JS apps are still the wild west when it comes to patterns that work well -- there are plenty of fertile experiments to go around. You want a library that gives you a foundation, so that your app can rise up to meet it and extend it in new directions. I think that pretty much all of the apps in this list fit that description -- going far beyond what Backbone provides out of the box: