One of the main reasons for using Ajax in any web application is
1.Rich User Experience
2.Reduce bandwidth
But a MSDN article I came across tells us that Update panel control does not reduce bandwidth usage in any manner (contradicting the general opinion about AJAXified web apps)
rendering shockingly easy and “for worse” because its simplicity and ease-of-use come at the cost of efficiency and, ironically, bandwidth.
While UpdatePanel brings AJAX magic to ordinary Web pages, it doesn’t bring the efficiency we normally associate with AJAX. Did you know, for example, that when an UpdatePanel control performs an asynchronous AJAX callback to the server to update its content, the request contains everything a conventional ASP.NET postback contains, including view state? Most developers assume that AJAX eliminates view state. That’s not the case, though, with UpdatePanel’s brand of AJAX.