This is a very relevant alternative view. Money quote:

At Zynga, however, Mark Pincus apparently likes to do things a bit differently. Rather than simply firing under-performing employees and handing unvested options over to the replacement, Pincus often likes to find another position within Zynga where the employee might still be able to contribute. But because that new position was often lower down the corporate totem poll, Pincus basically wanted to cut the person's compensation by reducing his or her number of unvested options (vested options were not touched).