Wrong - the auto-update feature is new to 7.x, so actual sites running 6.x (or earlier) doesn't benefit from it. Also, the new feature will only install and update modules and themes, not 'core', so I'm guessing updating core from 7.0 to 7.1 is probably going to require about as many manipulation steps as you need to go from 6.14 to 6.15. I don't have a lot of experience, but I would not say it's "extremely easy": the stable 6.x version requires you to do a lot of file and configuration manipulation just to go from 6.14 to 6.15 (if you follow all the recommended steps, which I did for my test site recently). It's not hard, it's just not automated.
If you set everything up properly, you just overwrite everything not in the sites directory with the new files, and then possibly run the database update script. Quite trivial.
I actually set up Drupal sites by symlinking to a main drupal installation for files and common modules. All the benefit of the /sites directory without having to use a single root favicon, deal with script-loaded sitemaps, etc.