It was more a comment about how considerations such as this make collaborative recommendation systems precisely less useful. Regarding rec.systems that move beyond naive rating&#x2F;like intersections between users (such as e.g. Jaccard similarity index), I think there could be interesting algorithms developed that make use of assigned&#x2F;curated item tags together with collaborative filtering&#x2F;recommendations (e.g. something that last.fm has considered (or has been considering) - there&#x27;s an entire field of research for just music recommendation systems.)