Other than the potential of injury, I think he's absolutely right to keep playing as long as he loves playing football. If he retires now, he'll never get to play NFL football again. As long as teams want him, I totally get why he keeps playing. How he plays now in no way diminishes his achievements from earlier in his career. I just hope he doesn't get seriously hurt. All the best to him and here's hoping he does well in Kansas City.
Ya, I would just be worried about his head with the concussions he had during the 07 season. Hopefully he is not forced to retire like Steve Young and Troy Aikman did because of Head injuries.