Absent the plea bargain option, many defendants would not have been charged in the first place.

I recall an incident about 10 years ago.  In a state where I was then practicing, not-so-minor traffic violations were often plea bargained down to a lesser violation, rather than force the cop to come in and the prosecutor to try the case and the defendant to be exposed to the greater penalties.  Since, for the governmental attorneys (and the judges) theirs was pretty much a part-time job, they had extra incentives to settle.  It worked like this:

Say, the defendant was (correctly) charged with running a stop light.  This was a 4 point offense, in a system which both allowed only 12 points to be accumulated before the operator's license would be suspended, and in which the auto insurance industry imposed a significant addition on the premiums of those with points.  Those additional premiums would continue for nearly 4 years.  In point of fact, the cheapest part of this ticket would be the court's fine.

To prove the running a stop light charge at trial would require bringing in a cop to testify, and a good half-hour to hour of actual trial time.  The courts would schedule a hundred or so of these cases every week, and more-serious cases (DWI, for example) would take precedence.  Thus, the minor cases (running the red light) would be rescheduled again and again.

There also existed a traffic offense called "obstructing traffic", which carried zero points and which had a pretty broad definition in the statute.  It was, therefore, standard practice for prosecutors to accept just about any factual basis for a guilty plea to the "obstructing traffic" offense, just to clear the dockets.  It got to the point where the state's Supreme Court (which had supervisory jurisdiction over the traffic courts) came out and said to the traffic courts "hey, you're accepting factual bases on this obstructing traffic offense which have no relation to the statute.  We decide that you will no longer accept plea bargains to this offense."  That limitations on what plea bargains the traffic courts could accept was pursuant to a well-worn precedent, and unchallengable.

All of a sudden, defendants who last week could plead their running a red light down to obstructing traffic - and turn a ticket that would cost them $150 for the fine and costs plus four points and a good $2500 to $3000 over the life of the insurance surcharge, into a ticket that would cost them $500 for the fine and costs plus no points and no surcharge - stopped pleading.  The subset of bar association members who spent most of their time in traffic court went public with their associational decision "We will not plead any of our cases.  We will try them all."

Prosecutors and traffic court judges went into a conniption and started threatening.

The defense bar went through a week or so of forcing every case to trial.

About 2 weeks later, the Legislature passed, and the governor signed into law, a bill (immediately effective) amending the traffic code, to provide a new offense called "unsafe driving" (or, as it came to be colloquially known "driving while stupid") which had a really, really broad definition (and thus would accept just about any factual basis) and, as importantly, bore zero points (for the first and second-time offender) and had a slightly higher fine.

And driving while stupid took the place of obstructing traffic in the lexicon of resolving matters.  And pleas once again began to be entered and cases resolved quickly.  

So, collective action by defendants in forcing prosecutors to try every case does, in fact, work.  I've seen it in action.