When developing a website or web application that is utilizing a base framework or system, it is very frowned upon to do any modifications to to the core system. In the case of Drupal, just like any other open source project, it is actively developed and released to the public in versions which adds new features, along with bug and security fixes. The issue of "hacking core" goes beyond just the core of a system but also to any of the possible add-ons that are developed by a third party. In the case of Drupal these can be modules or themes.