The system works pretty well in normal criminal cases. If the defense can show the subpoena was issued illegally the government can't use any evidence gathered as a result. It's a pretty big embarrassment for the cops and the DA, and doubly so if the suspect walks.
I think the argument people have is that the US Marshal service could abuse this mechanism to blindly issue reams of subpoenas and see what sticks.
The law should protect against that by requiring good cause to obtain the subpoena. Whether or not it does is the sticking point.