I'm going to throw my opinion/view in here: Basically back in the mid 2000's there were 2 things that spurred the all-js apps: Google docs and the mobile phone biz. Google showed everyone what a full-on js framework could do in a browser, but there was no available API for people use to build their own tools. Then when phone apps became all the rage, people started developing js/app APIs starting with sproutcore way back when (now called ember.js). Then there's sencha touch, etc. Backbone, etc is sort of an evolution on these themes of having a front end API but for the web.

In terms of going backward, I think that there are some advantages and drawbacks. There clearly a polling/ajax problem when alot of people need to be pinging the server for tiny updates. If all of this can be pushed onto the client (especially requests for html blobs) you reduce server traffic and make things more snappy (no more preloaders). The downside is having all your code on the client. I wonder if there are unsolved security implications to having so much logic on the client. All-in-all for a standard web app, theres nothing wrong with the old school 'keep it on the server' approach. But if you need alot of dynamic "app"-like behavior, then having a front end API will benefit you more in the end.