What are valid use cases for Salesforce Communities in Salesforce?

3 Significant Use Cases for Salesforce Communities
In my CRM/PRM consulting assignments, a question that I am usually asked by the clients is "What are the typical uses cases for deploying Salesforce Communities apps?" While there are several use cases that can be listed, I have seen three "umbrella" use cases that regularly appear:
1. As a community forum for interactions among employees, partners and customers:
The app is set up as a forum for your customers, partners and employees to collaborate / chat / share information, documents, and pictures - help foster greater engagement with your company and its brand. The focus is on seamless exchange of knowledge, ideas, and clarifications (questions/answers). Can be monitored and facilitated by a formal community manager appointed by your organization to drive the right message across the various communication channels available to employees, partners and customers.
2. As a front end web portal to automate select business processes which are supported on the back-end by solutions on the Force.com platform:
The app is a web portal to engage your customers / partners for specific business processes that are enabled partially on the app and partially in the backend (on a platform like Force.com). With self-service a primary driver, the user interface can have a custom look and feel that is unique to your brand. Also, it enables you to expose only a subset of the Salesforce objects that are relevant to the respective business process being automated.
3. As an alternative to applications on the Force.com platform i.e. to automate key business processes directly in Salesforce Communities:
The app is a comprehensive automation solution for entire business processes. For example, it is possible to enable partner relationship management processes like partner onboarding, lead distribution / management, market development funds and so on. Depending on the complexity and uniqueness of the processes being automated, there are AppExchange products (Salesforce Labs, as well as third party apps) that can be integrated to enhance and enrich the overall solution.

The valid use cases for Salesforce Communities areA) Internal users can use Salesforce automation without paying license fees.
B) External customers can have full user rights to Salesforce.
C) External partners can track the status of joint Opportunities.
D) External customers can track their purchases and open support cases.