Very often, plea bargains allow the defendant to agree to perform some socially beneficial tasks for the purpose of deterring similar crimes which are more beneficial to society than traditional incarceration. For example, those charged with littering might agree to clean up highways while wearing signs publicizing the embarrassing consequences of littering or those charged with petty thefts like shoplifting wearing signs identifying their crimes publicly in lieu of ninety-day jail sentences. In other instances, plea bargains in which defendants agree to conditions including reimbursing victims of their crimes also serve more social utility than imposing a term of incarceration without any restitution to the victims.

Besides the issues of cost savings associated with plea bargains in lieu of trial, incarceration itself is tremendously expensive to administrate.

Furthermore, in some cases, defendants convicted of relatively non-serious crimes could be seriously harmed by incarceration alongside dangerous criminals.

The Problems with Plea Bargains:

The principal problem with plea bargains is that where they are not controlled carefully, they can result in the premature return of dangerous or career criminals to the streets, where innocent citizens are exposed to unnecessary dangers. More fundamentally, many of those opposed to plea bargains suggest that allowing criminals to bargain for a reduced charge conflicts with the...

Specifically, the concern is that aggressive prosecutors could railroad innocent defendants into accepting plea bargains in lieu of prosecution for crimes of which they are, in fact, entirely innocent.
The Comparative Merits of the Arguments:

As in many areas of ethics and law, the ultimate value of a concept like plea bargaining is not as much a matter of black or white "right" or "wrong" but depends largely on more subtle elements such as the manner in which the tool is implemented. In that regard, the blanket use of plea bargaining to all criminal defendants without any consideration of the consequences would pose as many problems as the withholding of plea bargaining from all criminal defendants. Generally, plea bargains must be used judiciously with adequate appreciation for their social value where their benefits outweigh their risks. By offering plea bargains to some defendants while withholding them from others based on the many relevant factors, the mechanism of plea bargaining serves to reduce the monetary costs of criminal justice administration without any associated non-monetary costs associated with their indiscriminant use merely for administrative efficiency without any other considerations.