As it presently operates, probably no one. But I don't think the fault lies in the institution--I think the fault lies in the fact that the grand jurors don't realize what their true role is, i.e., that they are supposed to act as a check on the prosecutor.

My often-espoused theory is that in today's society we don't have role models for what is involved in being a grand juror. (A hundred or two hundred years ago, people had enough familiarity with grand jurors, which were an intrinsic part of their daily lives, that they knew what grand jurors were supposed to do.) The role model we have is for trial jurors, and trial jurors sit passively and do what the court tells them to do, which is pretty much listen to evidence and then vote. Consequently, I think people who are sworn in as grand jurors think that's what they're supposed to do, so that is what they do: They sit passively, listen to the prosecutor and then vote. If we can come up with some way of educating people so they know that the grand jurors are in charge of the grand jury and are supposed to run the show, acting as a check on the prosecutor, then I think the institution can begin to perform its intended function. 

I think trial jurors are very conscientious in performing their role. I think grand jurors would be, too, if they had a better understanding of their role, complete with its powers and responsibilities.