I think the answer have been said in the comments, its all about caching. Everytime we talk about high traffic and memory consumption for servers the first thing that came in mind, not only with AJAX but even for all other applications, is caching. But you'll ask in what requesting cached data will be useful for AJAX ?

I have experienced high traffic problem in my website for example, let's take the case of syndication file which was generated by server everytime. Even that this was not using too much memory, but multiply it with the number of users online it will be so much ! And the solution was to generate a static XML file everytime there is something new. Requesting static data from the server will solve the scalability problem and it does.

The same could goes for AJAX, if an application will have high traffic the most easy and efficient solution is to generate static files in the server side that the client will request everytime. It could be even session based, but could have other security issues. But the most important is to have in the server side the necessary tools to update this static file or regenerate it.

This is the way to get more performance with less ressources, we could also scale the servers to support some applications by distributed architecture, or other more complex systems this will cost more compared to the optimizations that could be done on the application level.