[support] Why SubTheme?

[Fred Jones]

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.