I guess I don't see how there is ambiguity in the on court off court numbers. For the season, the team is -9 points with Rondo on the floor. Per 48 that is -0.6. This is a very slight negative but essentially, it doesn't seem to make a difference whether he is on the floor or not.
If you go down through the 5 man units, most of the units where Rondo is out there with KG are strongly positive and most of the units where Rondo doesn't have KG out there are negative. KG overall is +90 and Pierce is +27.
What does this mean? It means that Rondo is a good player with holes in his game. I think it also it suggests he is inconsistent in his contribution to the team. Flashy assists in one game (or part of a game) and turnovers, missed shots, and porous defense the next.