Hashbangs are just a hack while we are waiting for the browsers of the world to all support push state. It is easy to use, well supported, and works really well. What twitter was saying is that in a public, content based app like theirs, the trade off of using that hack is not worth it, so they are moving to push state, and degrading to a worse experience for browsers that don't support it. Twitter isn't an argument against js apps, it is an argument against js hacks to provide fancy functionality to old browsers.
But then we need to use the same set of templates on server-side (on full page loads) and on client-side (when updating via JSON) ? Or we do like Quora and generate HTML on the server-side?
I'm not saying I agree with twitter, was just trying to explain their argument :) I think it is better to go fully one direction or the other. Either don't support IE9-, or go full reloads until they feel comfortable not supporting IE9-. (or stick with the hash bangs)
In a more general way, I use backbone to make data driven components. Those components are always rendered client side, and the layout/static content is rendered server side. I think duplicating would be theath to madness. Generally, it's fine to bootstrap initial data on page load, and render everything. But in times where that takes too long, I have rendered a "dead" version on the server (like, greyes out with a spinner) then replaced it on the client.
