The only bad thing about him not going to voluntary workouts is that the other players won't be able to see how he works. But it has nothing to do with Suh's own work ethic. The guy works his ass off and you can tell because he never gets hurt and pretty much never even comes out of the game. Very few defensive players take on the kind of snap load that he does. And he plays hard. If anything he plays too hard.
If you signed Suh to be "a leader" then you should have done your homework, but that's not the kind of guy he is. Suh is quiet, quirky, intellectual and keeps to himself. That's the way he's always been and it's the way he always will be.