Yeah, we totally understand the upgrade concern. The short answer about upgrading is you really don't have to, since Foundation isn't like wordpress or Drupal where there are security implications. ZURB has about 30 properties around the web and some are running on Foundation 5, 3, 2 and even pre-releases on 1. If we think the property will benefit we'll upgrade we will if not we won't.
The main benefits here are to file size, customizability and navigation patterns. We're doing a client upgrade now because they wanted to benefit from some of the additional Sass grid tools and then eventually be in a place where they could swap to the Flex grid and shrink file size even more.
Overall we'll upgrade some sites, but not all.
Hope that helps :)