I wrote a similar program but uses zones as well. All my links look like http://website.com/click.php?id=12&z=5
This way I can create many zones for different locations on my website and I can see where all my clicks are coming from. Tells me what banners and locations perform the best.
Capturing the referring URL will tell you what page your click came from but if you have many different banners on that page then it would be nice to know which locations are performing. Well… I guess you could figure it out if your not rotating the same banners in different locations.