I used to think that and I think there's an argument for that.
That said, Sullinger's post game might actually be more useful with the starters. KG and PP are increasingly having a hard time find their shot and Rondo is inconsistent in that area. Bass is someone who has to have his shot created for him. Sullinger on the other hand, could actually thrive in the half court with Rondo, PP, and KG feeding him, and PP and KG helping to space the floor.
Bass, while having potentially a harder time finding his shot, is better on the break than Sullinger, and could be part of a dynamic running unit with Rondo and Green later in the first quarter.