The difference between "The Dragon's Den" and having a potential client approach you with work is that the Dragons are being asked to _partner_ with the client,

The difference between "The Dragon's Den" and having a potential client approach you with work is that the Dragons are being asked to _partner_ with the client, not do work for them. If a potential client come to you with a questionable business and asks you to do work for them _as an investment_, you probably should walk away. If they want to pay you, then do what you think is best.