Late billionaire Robert Wilson refused to sign Giving Pledge, and the reasons why are detailed here in a 2010 email exchange with Bill Gates.
"Robert W. Wilson, the hedge fund founder who committed suicide at age 87 over the weekend, was known as one of the most active philanthropists in the country. But his style of giving was radically opposed to that of Bill Gates, who asked Wilson to join his Giving Pledge for the ultra-rich — and was rebuffed in a series of caustic emails. In emails from 2010 provided to BuzzFeed by a source close to Wilson, the hedge fund manager tells Gates that the pledge is essentially 'worthless' and that he wants to 'stay far away' from his effort." -- Rosie Gray, Buzzfeed.com