The ability to get a batter out with the bases empty (Grade) is a completely different ability than when a pitcher is in a pressure situation with runners on base (Clutch). Some pitchers just completely crumble when they get runners on and the game needs to reflect that. Without a clutch rating, the simulation couldn't take into account the "pressure" factor that the pitcher has to face when he's working from the stretch with a guy ready to score.

Jon