The aim of AJAX is the increased responsiveness of web pages achieved by exchanging small amounts of data with the server “behind the scenes” so that entire web pages do not have to be reloaded each time there is a need to fetch data from the server. This is intended to increase the web page’s interactivity, speed, functionality and usability.
AJAX is no silver bullet though, and the cost attached is the HTTP request count. Netvibes, for example, not only fires an HTTP request for every individual feed, but every single action as well. So, if you use Netvibes with 500 feeds, load the feeds, expand the feed widgets and mark all of them as read, you have accounted for a whooping 1500 HTTP requests.
Apart from the unnecessary server load, surfing Netvibes from places with uncomfortable latencies (like India) is a pretty unresponsive experience.