AJAX can be implemented in many different ways.

Options:

• Place all the interface logic on the client side, and have the server act only as backend functionality. For a complex application, this method may be prohibitively difficult. The overhead of placing all the logic on the client side, and managing its delivery and execution is probably not worth the performance gains.
• Segmenting the pages and populating them with HTML and related logic from the server. This would be using something similar to dojo content panes, which are HTML content. Similar to iframes.
• Half JAX. Run the pages as HTML templates, augmented with AJAX navigation/population. Thus, the HTML remains static for the page, but the AJAX is used to fill the fields and provide advance features. With an efficient template engine and a sophisticated architecture, this is an excellent approach.
• Use straight HTML pages, with a template engine. There is no law that AJAX must be used. :)

Delivering the HTML, with AJAX for form population and rich interface features is a nice compromise. The pages remain fairly fast, the rich features are available, and the complexity is limited. This is best implemented with a good architecture of reusable/common components.