This is an interesting topic to say the least. Very seldom do we see a player performing well to our knowledge, only to have a thread suggesting his play leaves something to be desired.
To the OP's credit, his observation is accurate. However, I feel that Rondo playing more aggressively, while calling off guys in the offense is how he functions best. I also think the big 3 wants him to be more assertive because in all fairness, Rondo's as important as anyone on that team. And when he takes it to the rim, he is getting to the line (when he's fouled) and making his shots. KG and Perkins (sometimes) set very good picks for him so when there's an open lane, what would you rather have him do, take the easy shot or make an extra pass?
When Rondo goes with the latter option, you usually see a missed, forced shot, or a teammate being called for 3 seconds because he waited too long to get the shot off.
Another thing, just because Rondo is a PG doesn't mean he has to be a playmaker all the time. The man is a scorer. You've been deceived by NBA "analysts" who insist that his role is best served being a "glue guy" who finds open guys for shots since he is not really a great shooter or scorer alone. This thinking is so wrong. Having watched Rondo for almost 2 and a half years here, it's obvious that he's a scorer more than anything else. I never said he couldn't pass. His passing ability is okay. But his scoring ability, when he chooses to take advantage of it, is pretty freeking good.