I have seen Rondo isolating a lot, but often he kicks it to Ray for a pretty decent look. Usually this is at the end of a quarter. If the game is on the line, it is usually in Pierce's hands.
But here is a hypothetical situation that might explain the reasoning: Imagine a line up of Rondo, Ray, Pierce, KG and Eddie House. Rondo has the ball at the top of the key, KG is somehow involved setting a screen and the others are spaced around the 3 point line. Rondo has the option to drive by his guy, do a pick and roll/pop with KG or kick it out to a 3 point shooter if he can't create anything. Also, with the pick, he could give it to KG who could pass to any of the other 3.
If PP or RA had the ball in the situation, they would try to isolate their guy and take a contested shot (which Pierce often makes, I know) or if they can't they'll pass it out for an equally bad shot. Having Rondo off the ball means that 1) his guy will sag off him because he is not a threat and 2) if the ball gets kicked out to him with 1 second left (because his guy is not covering him tightly) he'll have to shoot it and probably miss it.
At least in this situation, I'd rather Rondo have the ball because there are so many more good options that the defense has to consider.