I am flying quite often, but never signed up for frequent flyer miles card. I am skeptic about my privacy. I also don't understand very well how frequent flyer miles work.
With a search engine I found almost no info about this topic:
The major airlines actually make more money with the frequent flyer programs than they do as airlines. The market value of American Airlines frequent flyer program is more than $6 billion -- the market itself is $2.4 billion. What does it tell you? Airlines make a lot of money by selling miles to their miles partners -- credit card companies, banks, etcetera -- so you get a dollar for every dollar you spend
I always register with my real name and credit card at the airline. They anyway have this data and can they sell it?