The reason for this is that, I need write many detailed specs including content type definition and taxonomy term definition. And all the these specified need validated by customer. Often times, we need got customer to update or give feedback on certain kinds of questions; as business logic get clearer and clearer, we will have many times of updates on the content types, fields, taxonomy terms and so on. And during the process, the site architecture gradually take shape; only however, it happens only in my head, and I need put all the information stored in my head on paper correctly and in time (that is, before I forget anything :) ). In fact, Drupal is already a perfect tool for store and record all these things. We can use it as a dynamic prototype tool and document management tool. Customer even can get the real time update on the content types if they want.So, use Drupal as a direct project prototype and project management tool is a good idea!