We have been talking about this concept for over 20 years and have created significant new opportunities for clients to capture missed opportunities.  There is a lot of chatter in the Search Community related to “Topic Clusters” advocating a move away from more traditional keyword-focused efforts. I agree with some of these ideas that you do need to focus on the topic especially if you are the category leader.  Below are a few examples of research we have done for clients to help answer the question about how authoritative they actually are in the various categories related to their product portfolio.
For example, the highly competitive category of pet food. BTW, our research found that only manufacturers use this prase as most people need dog or cat food. Let us say for example that you have a broad portfolio of products like Purina that would be represented by 2,700 phrases. You have identified 500 phrases related to your dog food products and 300 for cat food and maybe another thousand for other types of pet food.  As the category leader would you not expect to be at the top of the search results for all of these phrases? In a consumer’s mind the most authoritative on the topic you would expect to be on the first page of the search engine for all or at least most of these phrases. Are you even monitoring this? In the analysis below, of the 2,700 words in the portfolio Purina was only represented 170 times. It was surprising that none of the online retailers were doing much better. The chart below shows of the words in the cluster how many times a specific domain appeared on page 1.