Let's just take a look at this from both the coaches and player's perspective.
You're Ben Roethlisberger, you were hurt and went down to get treatment, but the staff cleared you for whatever reason to go back up and into the game. You go out there and the coach is informed you are available.
The coach, for whatever reason holds you out. You watch your backup struggle mightily. At this point, you have a choice. You're Ben Roethlisberger. You are one of the best comeback QBs in the history of the game. You feel like this is a great opportunity to take control of the game and put it away. On the other hand, you also realize your coach is a moron. You know that you MAY go in and struggle. Or you MAY go in and do everything in your power, but the defense and coaching decisions are truthfully likely to leave you standing on the sideline at the end of the game.
So - Do you take the risk of completely ignoring your coach's call to hold you out and run out there, humiliate Dobbs and piss off Tomlin on the off chance you may be able to overcome both the Raiders AND Tomlin's idiocy?
If you're Tomlin - You're sitting there with a 4 point lead and a QB who was banged up. He's apparently good to go, but this is the Raiders, for God's sake. And you're really really tired of hearing that you are nothing without Ben Roethlisberger. At some point Dobbs has GOT to get some real game experience, so why not against arguably the worst team in the league? You've also got the cheaters coming up and THAT's a huge game. We need all our guys in as good of shape as we can get them.
And hey, if the worst happens? We'll absorb the negativity associated with our current situation.