What are appropriate use cases for Drupal as opposed to a lower-level framework like Ruby on Rails?

Rails is a general purpose web application framework for Ruby. It’s designed to help programmers be more productive in building web sites of all types, not just CMSs. And note that it’s for “programmers”. Unless you intend to write server-side code, you can’t hope to get much done with just Rails alone (but if you do intend to write code, both Ruby and Rails are known to be very productive).

Drupal is a Content Management System, of which there are hundreds. It happens to be written in PHP, but it’s modular design and large collection of available modules and themes make it possible to design and implement a large variety of web sites without writing a line of code. Most importantly, though, it is fundamentally a CMS. The further your site’s needs are from the CMS sweet-spot, the less likely it is that Drupal will be your best choice.