You're not as effective when you have something bothering you like a knee injury and trying to play at a high level. It's not like you're playing pickup ball at the Y or CIS ball. This is the highest level of ball in the world. This team is such a mash unit, it's not like they can afford to rest Bargnani in order for him to recover.
Then again, in terms of Bargnani, I can see his knees getting in the way on the offensive end because if your knees are not in your shot, chances are, that ball isn't as likely to go in the hoop. However, have we ever really seen him play with emotion or any real effort? Even when healthy healthy, it looks like he doesn't put out much effort on the defensive end anyway.