I have a (good) friend who thinks that as long as these topics are discussed in the abstract, there isn't much risk attached. It's true, the freq flyer tips are becoming dodgier, but this is when you are approaching maestro (ie Rudi) level. And sharing information doesn't mean acting on it, so the airlines needn't worry about a rush of phantom passengers/'business' travellers in the economy bathrooms, etc.

The travel agent who defrauded Delta was deliberately trying to harm the airline, whereas Rudi's phantom friend would give up the seat for paying passengers. Unless you take the airlines' money for a stunt like this, I don't see much harm done.