I was a prosecuting attorney for 12 years in a jurisdiction considered to be a "no plea bargain" system. In plain english, what you weree charged with was what you went to trial on or admitted to. For a prosecutor to change a charge, he or she had to explain to a judge the factual reason for the change, and the judge could approve or reject the change. On the good side, as a prosecutor knowing you were going to have to prove what you charged, you made sure you had good, appropriate charges. On the down side, our office went to trial more often than some of the big city prosecutors.
Now that I'm on the bench, I've seen the system change. The prosecutor and judges are allowing some plea bargaining. I can see the effects. Charges aren't as solid, and some are pure garbage thrown at a defendant to up the ante, so to speak, and push for a deal. I see wasted court time, because many defense attorneys will reject all deals until the morning of trial, at which point the best possible deal will be offered to avoid trial.
That's what I see as the biggest problem with plea bargaining...the issue of guilt/innocence becomes irrlelvant. Instead, it becomes a question of what can both sides agree on. I think the system suffers as a result.