when to use Drupal

In a way Drupal is only limited by ones knowledge and imagination.

I have used php, a CakePHP competitor called codeigniter ( similar also to zen framework) and Drupal to create database driven websites.

Codeigniter, like cakephp and zen, has many functions and libraries written in PHP to do common things like authentication, database CRUD (create, read, update, delete) and much more.

You use these libraries to create web site components such as a full login system, a form with validation, searches, etc.

See codeigniter library here: http://codeigniter.com/user_guide/toc.html

In other words, you could say that these frameworks combine PHP functions and code to create libraries that you combine to create a web site.

So these frameworks are a huge step up from PHP.

There is no reason for your developer to write your site in PHP, since then he/she would be rewriting the functions and libraries already available in Codeigniter, cakephp and zen.

It would literally take 100's if not 1000's of person hours to write the equivalent of these libraries, depending on which are needed.

Drupal is another huge step up from Codeigniter, cakephp and zen in that Drupal has already combined authentication, database CRUD, validiation, etc. into specific modules.

For example, CCK is Drupal's module system that automatically generates forms, and does the validation, CRUD for you.

Similarly, Drupal's Views module/system automatically creates search forms and does the database queries, and results for you.

In other words, you could say that Drupal combines various libraries and PHP functions to generate a portion of a website, such as a CRUD form, a search form, authentication, etc.

Again, it would take 100's if not 1000's of person hours to combine libraries from Codeigniter, like cakephp and zen to create Drupal module systems such as CCK and Views.

So using Drupal gives you the advantage of saving many hours of coding, with the very active support community of Drupal.

Both CCK and Views are IMHO incredible pieces of software, that allow great flexibility. They are extremely well written, better IMHO than many commercial packages inc. Microsoft.

But if they can't do exactly what you want, usually they can do it in another way.

If they can't do what you need, you can create your own Drupal code in the form of modules, using Drupal's hook API.

Drupal's API has similar classes/libraries to Codeigniter/CakePHP/Zen:

For example, for a simple database query:

Drupal has:
db_query
See http://api.drupal.org/api/function/db_query

Codeigniter has:
$this->db->select('title, content, date');
$query = $this->db->get('mytable');
See http://codeigniter.com/user_guide/database/active_record.html#select

CakePHP has:
find($type, $params)
See http://book.cakephp.org/view/73/Retrieving-Your-Data

In other words Drupal gives you the advantages of many modules, automatically generated forms and searches, ready to go authenitcation/login, AND the capabilities of CakePHP, if you need it.

From your brief description of what you need I would say CCK and Views can do what you want without modification.