Some fans can't separate Mario the player vs Mario the person. It's as simple as that. He's a moody guy but the coach's job is to get the most out of his players. If Rex really thought he was that bad, he should have sat him. My guess is, that on a lot of this tape where he's not charging full speed ahead is partly rex's scheme to have the DL plug their gaps.
Having said that, it's also clear that he wasn't happy with how he was being used and did take plays off.
The fault lies with both him and Rex. But if I had to pick one to keep around, I would have sent Rex on the first flight out of town.