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.
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.