[support] Why SubTheme?

Here http://drupal.org/node/225125 one can find a few ideas why one might want to use a subtheme. But if I have a site that needs only one theme, it's just one site, and there's no real interest in playing with ideas of different looks, is there any reason to use a subtheme of Zen or Blueprint as opposed to how I always do it and just make a fresh, 'regular' theme? Thanks.