Should police be consulted when plea bargaining is used?
No, the police should not be. This not the proper role of the police. The police do not have the correct perspective on the issue and do not know as much as the prosecutors do. One of the major reasons to plea bargain is because you do not think that you would be certain to win the case at trial. It is the prosecutors who know which kinds of cases are most likely to win in court. The police do not have the legal experience to make this determination. The role of the police is to arrest suspects and collect evidence, not to determine who should be charged and how.
The prosecuting officer is always involved because no one knows more about the case than he/she does. Particularly in guilty pleas, the DA is going to rely on the cop(s). It is their jurisdiction and community. They know the criminal element in the area, the harms that are being corrected, how dangerous or menacing the defendant is, and so forth. The prosecuting officer is going to know the factual strength of his case and probably have a good idea of any legal problems. So, yes the cops will be involved in a plea.
I agree, if we equate the police to a separation of powers doctrine, as the enforcers of law, they are seperate from the Judicial branch.