What we're really talking about here is an advanced form of caching. "Long lived objects" doesn't mean you do no validation, or that you don't hit the server to save something permanently.

The point is that due to the difference in architecture between an SPA and a more traditional all-server-side app, there's no reason to hit the server every single time. Isn't this the advantage of writing so much JavaScript? If you're going to make a bunch of requests on every page, why not just ditch the JavaScript and write it all server side?