Why should you create your theme as a Zen sub-theme as opposed to just modifying an existing theme?
Support: Providing or receiving support for any theme you develop in Drupal’s normal support forums can be difficult. The primary reason for this difficulty is that the person trying to provide support does not have any of the code you are using in your theme and/or they have never before seen the code you are using. If you have modified an existing theme, sometimes the only answer you will get to your support question is “Well, it works fine in the original theme.” Because you have modified the theme’s code in an unknown way, it will be extremely difficult to get help.
However, if you are following the Zen sub-theme guidelines for developing your theme (i.e. don’t modify any of Zen’s core files), any support issues become much simpler to deal with. You are modifying the STARTERKIT files in known ways. Zen core is carefully tested so many people know how it works and reacts in various browsers. And any differences you might be experiencing between your sub-theme and Zen core can be quickly isolated to override functions in your template.php or to override .tpl.php files.
Bug fixes: Occasionally there will be bugs found in Zen core. If you have modified any of Zen’s core files, it will be difficult for you to incorporate any of those bug fixes into your modified version of Zen. If, on the other hand, you have only modified files in your Zen sub-theme, getting those bug fixes becomes much easier; just replace the old Zen core files with the new ones. If your sub-theme overrides the PHP or CSS where the bug existed, the fixes will be carefully documented online so that you can more easily update your sub-theme.
New features: Occasionally there will be new features that will be added to Zen core. Again, as long as you haven’t modified any Zen core files, you can easily add those features to your sub-theme with minimal or no extra coding.