Click here to access article by Noah Feldman from Bloomberg.
This very mild critique of the use of grand juries barely scratches the surface. But, of course, that would be expected from a ruling class media source. I'd like to briefly, because of time constraints, offer another argument.
Grand juries are used whenever the ruling capitalist class has a strong interest in a particular case. This is so because grand juries are sometimes seen by our masters as a useful legal excuse to harass political dissidents, or prevent the use of a standard jury trial when a grand jury could serve their interests better. You see, a jury trial has much more rigorous standards to insure the operation of the rule of law to serve justice than does a grand jury investigation.
It has been rightly observed that "a grand jury would 'indict a ham sandwich,' if that's what you wanted" and thus this is why they are used to harass dissidents. But, of course, the Ferguson case involved a member of an enforcement agency that serves the ruling class. Hence, the need for the latter to protect this person, and therefore they decided to use a grand jury to accomplish this instead of a trial jury in which he more likely would have been found guilty of some crime.
For an excellent source on the operation of grand juries, I strongly recommend that you read this brief piece entitled "A Few Facts About Grand Juries" from Grand Jury Resistance Project. Grand juries vary a little by states, but I noticed that in the Ferguson case that a judge appointed the jury. This is obviously a great advantage from a ruling class point of view.