I prototype business logic in prolog by turning client requirements into predicates. This allows me to formalize their prose and to verify I understand it correctly by asking the client for expected results for a given input/scenario. These programs never make it to production, though; I rather see them as a shadow spec.

EDIT: IMHO one should learn it just to get introduced into the mindset of declarative programming. You can implement a sufficiently fast and complete prolog easily in any language if that allows for a clearer solution to a specific problem than explicit specification of control flow.