The hierarchy of the Community is Community Topic > Community Posts. I like to be able to create subtopics. Especially as long as Zendesk doesn't offer country-specific Communities, we need subtopics. Here's why:
Zendesk's current solution is to use topics to localize the Community. Meaning, we create a topic called "Germany", another called "UK", and so on. But then the user clicks on the topic and lands directly in the list of posts. But the user should see subtopics that cover the actual topics, like 'General FAQ' or 'Product Requests'. Without subtopics, there is no way to include the actual topics if you have already used the topics for localization.