There are several cases where you would want to break down your view into several small components. One use case that I am working with right now, is I have a multi-lingual site that I would like to reload content using AJAX principles.
Normally what I would do in the case of a non-multi lingual site is to create another ActionResult to return the ViewModel that is changing with the new parameters. I like to use a custom ActionResult that I have called JsonpResult. The problem resides in the fact that I have labels not in my database but in Resource files. So what I would need to do is to somehow hydrate my Resource file data into the ViewModel.
This definitely works, however it becomes a question of maintainability. I now need to not only maintain my original ASP.NET view, but I also need to maintain a set of scripts that handle AJAXian behavior. If you need to have your site SEO, then you really need to make sure that both the Server Side and Client Side behavior are both working the same.
This is where Partial Views come into play for me. What I do is "pull out" the logical data sections where the bulk of the reload occurs. The nice thing about PartialView is that you can pass your ViewData and Model along to the PartialView. If your PartialView is strongly typed against your ViewModel you can get Intellisense to help with the wiring of the PartialView.
Now all I need to do with my AJAX call is to write the response back to a single DIV rather than handling data points individually. What it does mean is that there would be more content coming down the pipe. However, the trade off is easier to read and maintain code.