Many PlayStation 2 games which were programmed to use the analog functionality of the PlayStation 2 controllers buttons will not accept non-analog input therefore Sixaxis or DualShock 3 controllers must be used (though this could potentially be solved with future firmware updates).
Certain PlayStation 2 games programmed for analog controller use do not support non-analog inputs. Consequently, the Sixaxis or DualShock 3 controllers are necessary to play these games. However, future firmware updates might address this issue.