I once talked to a guy who had held a fairly low-level IT position at her company in Chicago, Harpo Productions. He and every other employee were forced to sign draconian nondisclosure agreements stating that they would “not reveal ANY personal information about Ms. Winfrey.”
Gotta have a whole lot of skeletons in her closet to insist on THAT.
Not really, if you're a billionaire.
Without an NDA, a disgruntled ex-employee (or just one hoping to strike it rich) can malign you in subtle ways, invent little incidents that don't rise to the level of "libel," but still damage your "brand."
Regards,