Live re-rendering of data -- imagine if the (now) "24 minutes ago" timestamp next to your handle was always accurate, instead of just reflecting when I loaded the page.
Easier implementation of otherwise difficult features. Doing a client-side autocomplete of people's names is trivial when you already have all of the other accounts in your organization modeled in JavaScript ... but not as fun when you have to ajax for server-rendered HTML for it.
Live updates of pushed data. When another reader +1's a post that you're currently looking at, if the server pushes that data to you, and you have a model for that post -- it's easy to increment the counter. If the server has to push the re-rendered HTML for the entire post, it's much more difficult.
This can be done without the radical switch to modeling everything client-side. I mainly do java web apps with JSP's (which everyone seems to think is stodgy and past its prime) but its rather simple to do something like what you're describing with keeping things up-to-date. I simply have broken my project down into "widgets" that self-load so a page actually is made up of many widgets that use javascript to call various server-side controllers, you can then set them to do whatever you want (such as refresh every 30 seconds) and it doesn't require some massive client-side data model.
I agree, there are cases where you would want a more robust client side data model, but, IMO people are a bit too quick to jump to using this and it can make more work in the end and make a project more costly to maintain. I think the cases where it is useful are mainly for highly interactive sites like those that would have previously done in flash, with animations (such as games, etc), for just regular CRUD webapps I feel its a bit of overkill
Easier implementation of otherwise difficult features. Doing a client-side autocomplete of people's names is trivial when you already have all of the other accounts in your organization modeled in JavaScript ... but not as fun when you have to ajax for server-rendered HTML for it.
Live updates of pushed data. When another reader +1's a post that you're currently looking at, if the server pushes that data to you, and you have a model for that post -- it's easy to increment the counter. If the server has to push the re-rendered HTML for the entire post, it's much more difficult.
... and those are just the tip of the iceberg.