The most important player on this team is the one who can play the best.
PP and KG were and if they are still able to play their best then either still are the most important on this team.
Rondo can have monster games, but he needs others to play well in order for him to have those monsters.
I'm guessing you're referring to the differences between scorers (Pierce) & defensive players (KG) vs. distributors (Rondo). Players have to play well, i.e. make shots, for Rondo to have a great statistical night, whereas Pierce can just go off one-on-one and take sole credit for all of his production or KG can spearhead a dominant defense.
But that really doesn't constitute much of a difference. Andre Miller once averaged 11apg playing with Wesley Person, Lamond Murray, Jumaine Jones, Ricky Davis and Chris Mihm. And don't forget the Nets teams Jason Kidd led to the Finals. Most NBA players can put the ball in the basket. That shouldn't diminish the credit given to the great distributors.