PB, the scenario you describe probably happens enough of the time. However, the purpose of a grand jury is to keep government on the federal and state level from just plucking people off the street and prosecuting them. The grand jury is sort of a gatekeeper or filter although, truth be told, all the resources are on the state's side (as they say, a district attorney can get a grand jury to indict a ham sandwich). So, on balance, the pejoratives that may affect "innocent" parties are outweighed by society's need for order and the rule of law.