Some application architects only want data-layer functionality in the server with a thin web services interface. In such a situation, it is useful to have the MVC architecture in the client.
At the end of the day, it depends where you want to put your controller and view functionality. If you want it all in the server use Rails without any AJAX. Having a model layer in the server lets you encode complex relationships, views, and validation in the server. It lets your controller respond to different data formats (e.g. xml, html, json). And it lets you use ERB or HAML to control the views.
However in many situations, you may want to offload processing to the client. In such a case, you may want to move view processing to the client (such as with AJAX). Or you may want to have the client determine whether to render html, xhtml, html5, or whatever. Or you may want to use some local storage to cache data in the client. Or you may want to do data validation and view composition in the browser.
Some application architects only want data-layer functionality in the server with a thin web services interface. In such a situation, it is useful to have the MVC architecture in the client is prudent because you'll eventually have to deal with model-layer issues such as validation, complex views, data filtering, controller-layer issues such as view formatting decisions, and view-layer issues such as layout, rendering, and styling.