Found this from the sitepoint link posted in a previous article:

Ajax is meant for those situations where you have a small part of a web page that you want to update with information from the server without reloading the whole page (often a single word or a small set of links). If you are looking to replace more than say 10% of the page then you need to rethink whether Ajax is the appropriate way to do it. At least some of the Ajax that people are currently writing is more so they can demonstrate that they can write it rather than that the page actually needs it. Once Ajax ceases to be flavour of the month then it will go back to being used only in those situations where it is appropriate (the way people used to use it several years ago before the name Ajax was applied to this particular technology).

(Posted by felgall – URL: http://www.sitepoint.com/forums /showthread.php?t=371856)

I thought that was an excellent summation.