Davey Graham is a self-employed musician, charging for his services and earning good money for his privilege. He has customers, and a duty to those customers in that he offers them his services in return for their cash. If he fails to deliver the goods as "fit for the purpose" then he fails to fulfill his contract. Customers have a right to complain - and if someone goes on their roof to fix a leak and he is ill prepared, no surprises then that the rain continues to come through. If you can't see the analogy, you obviously won't take the point anyway.