I used a program to delete all of my posts and comments at once. It took some time since it went post by post, but I didn't want any of my intellectual property making money for a company that continues to shit all over users.
On a technical level, it's absolutely true. They already have to keep it for legal purposes (e.g. a user posting illegal material, then deleting/altering it).
As for whether they're actively harvesting that for sale is another matter entirely.