I've just been building a large app with Backbone.js, and I can tell you that it is seriously versatile. I don't use the routing at all, I just move objects around and save and load them from and to the server. The great advantage is in the structure it gives your app.

Say you have an onscreen cat picture which can be moved around the screen (it has x,y coordinates). So you create a CatModel with attributes called x and y. You have a CatView which has an associated div - the cat picture. Now you can just bind your view to the "change:x" and "change:y" events of the CatModel, and have it update onscreen whenever the data model changes. Maybe every time you move the CatView with your mouse you call model.save() - that will be sent as a REST request for you. Now call model.fetch() every 5 seconds, and the onscreen cat picture will move to whatever state is held on the server. The code is clean, and tiny.

I guess what I'm saying is that it will only take you 5 minutes to create a massively multiplayer online cat picture moving game, with far less code you might otherwise have needed.