This bugged me too since it was pretty obvious after the Atlanta game that the knee was hurting him. Now, in Doc's defense, maybe Pierce lied to him about how he felt and it wasn't obvious how much the knee was effecting Pierce until the end of the first quarter or so. I mean not being there we are kind of jumping to conclusions on this one based on facts not in evidence.
We don't know what Pierce told Doc. We don't know if Pierce misled him regarding just how well that knee was feeling. We don't know what the relationship between the two is and whether Doc let Pierce out there to kinda prove a point that Pierce should listen to advice and stop being Superman all the time when it comes to playing minutes, especially with a deep bench. heck, not having been at the interview or heard the interview where he said it, I'm not so sure what the context of the remark was.
But, if Doc knew for sure that the knee was causing him pain and range of motion and let him in anyway based on Pierce telling him he wanted to gut it out, then yeah, I have a problem there. Comes a time in older players careers when you have to protect them from themselves for the greater good of the team. Now, just how much more damage could have really been done to a bruise is debatable but still, injuries from overcompensation can happen and Doc has to realize his team is deep and they can survive with giving the vets off a game here or there to rest a nagging injury.