Tony needs people around him that are not afraid to tell him he's wrong and well shut up and listen. Zippy has that ability and I think Steve does too. Part of the reason is that they are both secure in NASCAR, if Tony would be foolishly stupid enough to fire them someone would be there to pick them up in a heart beat. I don't think Darian ever felt strong enough or secure enough in himself to do that with Tony.
I still remember Zippy telling Tony "that's enough of that crap". Tony may be the boss but I think he knows he needs people around him to keep in check too.
"I`m too lazy to work my real job. I love what I do." Tony Stewart