Ajax the implementation
The more I thought about that question, the less I understood it. I mean this site is certainly not slow (http://www.websiteoptimization.com/services/analyze) so I didn't need it for that. There's not that many options on each page, so I don't need it for real estate.
Then I decided to look around the web to see how all of these other people were using it. I found this page which gives some examples of the usage of AJAX:
So, you use AJAX if you have large amounts of data which are conditionally visible at runtime - such as Help Text, List Items, Maps :-) - and AJAX will help you to deliver this data in a "Just in Time" manner to help reduce the initial time to load for the page. This could be a winner for things such as interactive help and certain complex validation logic scenarios.
Another consideration that you should take into account when supplying AJAX interfaces is whether or not the loss of the back button is a good or bad thing. In some cases I find it extremely useful to be able to "back" navigate through my previous page selections whereas in others (such as complex management screens) I would not miss it at all.
Ever used AJAX in your own applications? I'd love to hear the thinking behind what went into the decision.