When Sully was first drafted, I liked this idea a lot and got behind him as a starting PF over Brandon Bass ... however, as I have had more time to think about it, I believe I was being overly-optimistic about Sullinger's ability to outplay Bass at the starting PF position.
Given his defensive limitations and in his rookie year where he is still learning the NBA game from a defensive standpoint, I think Sullinger will be too large of a negative defensively to out play Bass despite his superior offense + rebounding.
Better to keep Bass as the starter and use Sully as the primary backup PF where Sully can enjoy easier matchups against lesser quality PFs. I think that rotation gives Boston a more productive PF rotation.