Drupal is a CMS, not a web framework.
Does this mean it should only be used for publishing posts, blogs, graphs or what ever content you can think of or can it be used for more sophisticated applications with 'true' business logic behind the scenes ?
Can it be used for developing a resource management application? Can we extend the Drupal database with our own tables that represent business objects?
You should be able to extend Drupal with custom modules to achieve your business goals. I wouldn't add onto the Drupal database adjusting it's schema since that would make patching and upgrading difficult to say the least. You could use an additional database if needed if the existing Drupal database API doesn't meet your needs. There is also a Drupal services module available (https://drupal.org/project/Services) that you could use for a web API interface between your app and Drupal to access it's objects.
This Stackoverflow post also may provide some answers, Web application integration with Drupal -- I have been involved with successful back-office business applications that utilize a CMS such as Drupal extending it using custom modules and an additional database for custom data objects.
