A post over at Mister DA got me thinking about the reasons we plea bargain.
Off the top of my head I can think of three reasons: (1) Overcharging, (2) Judicial Efficiency, & (3) Uncertainty.
Overcharging: This happens more than it really should. In Virginia the charges are usually taken out by the officer without a whole lot of input from the prosecutor's office (exceptions to this are probably murder cases and other very serious, very technical cases). At times this is a single charge which is not supported by the facts. For example, in one jurisdiction wherein I practice it seems that almost every time a suspect drives away from an officer standing next to his car he is charged with attempted murder of a law enforcement officer. This charge does not stand up in court and we all know it. This will get pled down to something like an eluding charge.
At other times this comes from stacking charges. There is no prohibition under Virginia law against stacking except for the relatively minor "single larceny doctrine." Thus, instead of only charging Client with robbery the officer swears out warrants for several gun charges, robbery, abduction, eluding, jaywalking, & 7 traffic offenses. Client is guilty of it all but it's clearly overkill (this can be particularly devastating when charges that carry mandatory minimums are stacked). A more common form of this is to charge someone who has passed a check not her own with forgery, uttering, and grand larceny. Drug addicts will pass all sorts of bad checks in order to feed their habit; it's not all that uncommon to get assigned a client with 20+ felony charges (just in one county). The worst I've had so far was 52 felony check charges. The prosecutor agreed to drop all but 6 charges.
Judicial Efficiency: This is legal terminology which means "not bogging the court down so badly that only 5 criminal cases get done in the entire year." Cases like the 52 felony charges case above get plea bargains because the prosecutor knows what will happen if he does not: I'll make him prove all of them rather than recommend my client plead guilty. I'm doubtful he will be able to prove each and every charge. Who knows how many days (weeks?) that case might take? The prosecutor will have to go through the nightmare of rounding up all the clerks and have them testify as to each and every check and have I'd be there catching every error they made (hopefully). Unless the case is a political hot potato or he is angry at the judge the prosecutor will plea bargain in this case.
In more common case the prosecutor will drop or reduce one charge in order to get a guilty plea on the other. If he doesn't, and 20 Defense attorneys on a docket are suddenly trying their cases, a docket scheduled for three hours make take a day or two. In the long run this can be just as devastating as not reaching a plea agreement in the less common 52 charges type case.
Uncertainty: How is the case going to turn out if we try it? Even in fairly predictable jurisdictions with fairly predictable judges there is always that chance things might not go as expected. This is particularly true if the charge is unusual or the legal theory is creative. Will the judge buy the government's argument that a solid suspended in a liquid is a gas when its dispenser sprays it in a mist (for a statute forbidding the release of noxious gases)? The Commonwealth thinks the usual pro-conviction inertia will carry his innovative theory across the finish line. I think the judge will have taken high school chemistry and know the difference between solids, liquids, and gases. Neither knows for certain how the case will turn out. Usually this will result in a plea agreement for a lesser charge.
Conclusion: Seldom does one of these factors stand alone as the reason for a plea agreement. A perfect example of this is the 52 felony charges case discussed above. It was definitely overkill on the number of charges; trying that case would take a looooonngg time; and no one knows for how many of those charges the prosecutor would be able get the witnesses and proof together. Reducing it to 6 felonies gave the prosecutor a significant amount of convictions and gave my client an incentive to plead guilty. We didn't know how many charges the prosecutor could prove but it was very probably more than 6.