1. The use of AJAX the increases the richness and responsiveness of the web page interface.
2. It reduces the network traffic and CPU usage on web server. This is because there will be no post back to the server that will render a complete HTML page. For example, if you are displaying a lot of data in a web page, the HTML page size may be about 100 kilobytes. This big string has to be created on the server and then sent back to the client.
AJAX breaks the normal browsers’ Back button behavior. When a page is updated dynamically, returning to the previous state may not be possible, since browsers typically record only complete page requests in their history lists.
As not all browsers are complying with W3C standards, AJAX applications have to be tested rigorously to deal with the quirks of different browsers.