You make a very good point. The catalyst for major donations—eliminating the capital gains tax on private company shares—would be at a time when the entrepreneur is deciding to retire and the children aren't going to take over the business. The owner is going to end up selling the business. That would be a catalyst for significant, increased charitable donations. It would also give the owner of the private business the same tax treatment as the entrepreneur who starts a public company and donates his or her shares to charity.
On February 7th, 2012. See this statement in context.