The important part is PBCs protect you from a shareholder primacy directive. Eric Ries describes it in his new book, but the example he gives is if the most evil company you know tried to buy out your company you have to do it in a normal "best practices" C corp because it is your fiduciary duty. PBC helps prevent that based on your declared mission statement. If the sale doesn't facilitate your mission then you aren't obligated to sell.