Bargnani is a match up problem for a lot of other teams. Its why he's useful. There's no doubt that he's an exceptionally talented player.
However, because he's got a glaring weakness, the situation is reversed at times and the player he's guarding can take advantage of his D.
A player like that should come off the bench. Until he can play consistent D he's better utilized as a situational player.
Still, I wouldn't like to see him go, because there are definitely times when he can torch the other team and win a game for us.