The company may reserve a right to buy back fully vested shares.  Employees may decide not to work for a company if the scheme is too harsh.  Considerations will include (1) what the price per share will be at the time of buy back, and what the time period is for the company to repurchase.  Companies may take the stance that the growing company has issued shares for the purpose of working together to achieve a common goal of selling the business or reaching sustainability, and until the company gets to the point of sustainability, the shares need to be reserved for the employees who are still "in the trenches."