I've helped build a 100 K line single page app in ExtJS, and I can see the value ember would have for building that app (have not looked much into angular so ca

I've helped build a 100 K line single page app in ExtJS, and I can see the value ember would have for building that app (have not looked much into angular so cannot comment on that one). What you need in large-scale single page apps are coping mechanisms for the DOM (or as I know it: the root of all evil). Stuff like automatic and on-demand rendering of content, automatic cleanup and event listener removal, nested layouting solutions, strong eventing and routing mechanisms, data binding and automatic data loading, and so on. Any framework that helps you with those things by necessity will invent its own pseudo-DSL, and require you to live in its world. The way I see it, it's all a matter of what you're optimizing for. If you expect to run at most 2000 lines of code in a single page, then backbone is excellent for that. If you expect to run at least 20.000 lines of code in a single page, then something like ember or ext is needed. You can build those in backbone ofcourse, but you'll probably end up rebuilding much of the infrastructure the bigger frameworks give you out of the box. Ofcourse, this is separate from the whole debate on whether a pure client-side approach makes sense on that scale. I still think it does, but I think the mixed model is equally valid (different trade-offs in each case). You're right that the mixed model blends poorly with these frameworks.