Ajax does use very small amounts of bandwidth but you have to think of this in terms of how big Webforumz is. Let's say there are 50 users online, this means that every x seconds (probably 2 or 3) there will be another request sent to the server. This request will be sent even if the user wanders away from the computer with the page open.

Webforumz is usually busier than this so that really adds up over time with so many requests hitting the server even after a user has loaded a page. It is more a server load factor than a bandwidth factor.