Some of my lower traffic sites were still on an old Drupal 5, others were at 6.4 and lots of modules were outdated. So I set out to migrate all the sites to a multisite setup. I now have a set of shared modules , shared themes and some specific ones .. just as it is meant to be. And I only have 1 central Drupal instance to upgrade. Next step is figuring out how to trigger the database upgrades from 1 central place.