A good prosecutor uses a grand jury as a tool to evaluate the chances of winning a case at trial. It's essentially a focus group, which are now used frequently in civil cases. As with a focus group, a good prosecutor will present the problematic evidence to a grand jury to see how it is received, because that problematic evidence will later be presented by the defendant at the trial. An important measure of a prosecutor's job performance is conviction rate at trial (no one cares about indictment rate at the grand jury), so there are not many prosecutors who would want to skew the evidence in the prosecution's favor with a grand jury, only to go on and lose at trial when a good defense attorney presents all the problems with the case. Actually, I'd say the opposite is more likely. A prosecutor who doesn't like the chances of winning at trial can tank the case in front of the grand jury, and have an easy out when the boss and victims want to know why the case is being dropped.
As a former prosecutor, I have to disagree with this assessment of the purpose of a Grand Jury, and how prosecutors use them. Having spent many days presenting cases to a GJ, I can tell you with certainty, we never gave the "problematic" evidence a thought before presenting a case. And nor should we have.
The purpose of the Grand Jury is to serve as a buffer between the almighty power of the prosecutor's office and the rights of the People. The thinking has always been that the enormous police power of the state to charge an individual with a major crime (a felony; i.e., a crime with potential jail time of more than 1 year) shouldn't be absolute. So the idea of the Grand Jury is to serve as that check.
As several other posters have noted, the burden of proof on the prosecution in the GJ is much lower than at trial. Is there reasonable evidence to believe that a crime has been committed, and that the defendant is implicated in that crime to a sufficient degree to officially charge that individual and proceed to trial? A pretty low bar. I do recall one prosecutor in my office who made some kind of argument in court (after an indictment) that because of the strength of the indictment, the defendant was facing serious trouble. The judge's response, which was endlessly repeated in our office: "Counselor, you could indict a ham sandwich...".
So when presenting cases to the GJ, we presented only as much of the evidence as we needed to to ensure the indictment. We almost never presented actual witnesses or introduced physical evidence. It wasn't necessary, and was seen as counterproductive. What some of you might not know is that when you present a witness at the GJ, you must turn over the transcript of that testimony to the defense before trial. If the witness at trial says something even remotely different than what they said in the GJ, the defense lawyer will use that against them to damage their credibility. And since hearsay evidence is admissable in the GJ, there is usually no need to present more than one witness, usually the Police Officer that conducted the witness interviews. "I interviewed Vince Victim, and he told me that the defendant pulled gun on him and robbed him". Done.
My office didn't need to or want to use the GJ to do dry runs of trials; first off, is no time to do that right. Defendants that are in jail on bail have a right to quickly be indicted or freed. that means the police and the prosecutors have to be quick about GJ action on felonies, and that means no time to put together a practice trial. But more importantly, there is no need to do so. My former office (as do most other DA's offices that I am familiar with) already screen cases immediately after an arrest. Any case with serious evidentiary problems is pulled from the queue of regularly processed cases, and is either given more attention, or sh*tcanned immediately.
Most jurisdictions have a different kind of GJ as well: the investigative GJ. These bodies act independently (remember the phrase "runaway GJ?) and have subpoena power to their own investigations and issue reports and recommendations. But it does not appear that the GJ in question for this unfortunate Captain was one of those.
So, for those of you who think that the fact he was indicted means that the case against this guy is strong or weak, I have to disagree. We don't know anything from the GJ's actions.
And no one I know of ever tanked a case in GJ because we were concerned about our conviction rates. None of us knew enough about the case at the time of GJ presentation to ever worry about. And even if we did think the case stunk (and a lot of them did), the GJ process is more like a factory than anything else. We had no time or energy to worry about what would happen to a case later on; it wasn't going to trial for at least a year, and who knew what would happen by then.
In my five years as a prosecutor, I probably presented a couple hundred matters to a GJ; I remember only one time when I wanted an indictment and didn't get it. I also remember a few times when I wanted them to return a "No True Bill" (no indictment); those were more successful.