The indexer is responsible for splitting the infromation in terms and making it usable for a search engine. In a very abstract sense we want to have a maximally complete and grounded view of a product, on all aspects and in particular on the lifecycle of a product. That means that the indexer should also be able to weight the newly found information against the already existing information and consider whether or not it adds something valuable. The confidence in correctness might also be a considereation. This thinking is quite beyond a standard 'indexer'.