Here is my theory. I think it has to do with the fact that Rondo is at his best when he is able to go all out. He is very similar to Garnett in that way. So when you have a high energy player like that, by giving them significantly more minutes (especially over a season), you risk "watering down" those minutes. For players whose games are more based on shooting, and generally play slower, this isn't as important, but for a guy like Rondo or Garnett (who is given the exact same treatment, where he is on a strict minute limit), it can be the difference of being dominant, and ineffective.