It's there to plug a temporary hole and it's quite useful for that, at least for the time being. Other than that though, you can actually do pretty cool stuff with it.
What about those people that think Ajax is a dirty hack and you should not misuse the web like that? I think they should think back about how the internet initially got it's growth by hijacking telephone lines, something they were never designed for. It is a more powerful language than many people think it is. Yes, you can create Ajax applications with Wicket without ever writing a single line of Java Script.
Ajax is not the perfect everlasting solution to things, but it is a great trick for now and people enjoy slicker and faster end-user interfaces and improved productivity as a result of it. Until there is another universally accepted and available thin client, other than the browser, we will have to stick with dirty hacks! The autocomplete component for instance, which uses Ajax to pop up a list of choices in a text field while you type, can be used like this: As you can see, no Java Script, and in fact it works exactly like non-Ajax components work.
However, I think that the real cool thing about Wicket's Ajax implementation is that you can encapsulate such behavior in self contained components.
Also, since Wicket doesn't push state to the client, it is secure by default and the development effort may be easier (depending on what you're doing), as your model doesn't need to be transported anywhere. GWT produces something that is more akin to applets; independent applications that happen to be hosted on web pages. Wicket takes care of users having to marshall and unmarshall data between server and browser by working around the problem.
So Ajax is not terribly interesting in the long term. The worst thing about Java Script is that it is not statically typed, so if you need to maintain a lot of Java Script code you'll need to be very disciplined (and your team members as well) or you'll end up in maintenance hell.
That enables you to write Ajax components that you can embed in a page without needing any additional configuration to make it work.
The auto complete text field above is a good example of that. Learning Java Script is not hard, what is hard is learning Fire Fox Java Script, IE6 Java Script, IE7 Java Script, Opera Java Script, Web Kit Java Script, etc.
The differences are subtle but can cause huge hang ups and are very annoying to Java developers who are used to Java being much more consistent.
Another pain point is the lack of a really good Java Script debugger, which is a tool Java developers take for granted.