Backward compatibility for data, not code
Everyone considering Drupal should understand that Drupal development is always on the cutting edge, and with each major release there will be radical improvements.
Why ignore backward compatibility?
1. Each new major release of Drupal contains many, often radical, improvements in functionality, scalability and usability.
2. These advancements are made possible by not maintaining backwards compatibility with previously released code (however, stable and reliable upgrade paths are part of the planning for each and every release).
3. There is a path for updating your site with Drupal core.
4. Only the current stable release series and the previous release series (, 7.x and 6.x) are supported by the Drupal development community at any given time.
5. As a result, each major release of Drupal will eventually age to the point that it is no longer actively supported by the Drupal community.
6. Unsupported releases may, in the future, be found to be vulnerable to as-yet-undiscovered or yet-to-be-invented security vulnerabilities.