I have been an ardent supporter for Ajax technology. It can be very useful for websites where data loaded at a time is quite huge and we can break it up into smaller groups which can interact with Server independently and asynchronously. One of the prime example for this could be Enterprise Portals.
The other issue is how well these Frameworks are adapted for new technologies? Can they be used in Enterprise Portals where each portlet window can have same Ajax implementation frameworks and hence clashes of names, actionURL etc could be possible. Plus in portals the response is not send to the portlet at all. Can the framework adapt to such changes?