In State of Ajax: Progress, Challenges, and Implications for SOAs Dion Hinchcliffe discusses some of the benefits of the Ajax approach.
"Practitioners of Ajax get high-intensity user interaction (end-user productivity), asynchronicity (efficient backround processing), web browser access to web services (web service access, reuse, and interoperability, as well as SOA integration), platform neutrality (browser and operating system agnosticity), and the Ajax feature set can be delivered as a framework you don't have to create yourself (developer productivity)"
The one thing I would say this is missing is the ability to easily manage the business rules inherent in the user interface. For instance, if I want to ask a policy holder if they have hurricane shutters but only do so in certain states I could use an Ajax approach and code that in the user interface. But, how can my underwriters add new states to this list and how can I reuse this rule when checking the completeness of an XML feed from an agent, for instance?
One way is to use a business rules approach and adopt technology that let's me push these business rules into the user interface in an Ajax-like way. Then I can use modern business rules technology to expose those rules to my business users - solving my maintenance problem - and re-use those rules on both client and server. One such product is SmartForms for Blaze Advisor. I am sure it will not be the last.
I wrote a little more on this earlier - The Changing User Interface.