I disagree. Its more about making sure the system doesn't collapse. If everyone who was under arrest went to trial...that would absolutely destroy the court system. Think about it. The FBI says about 25,000-30,000 people are arrested every day. In a week, that would be nearly 200,000 cases that have to be heard. Now I'm not saying they will be heard every week but eventually with that sort of caseload...the backlog will dwarf any court resources available. I mean thats a almost a million cases a month, every month. There aren't enough district attorneys to do a fraction of that. So the plea bargain is more to move the system along than to protect someone's career.
Most times when the state goes to trial, they win. My own personal take on it is that the jurors--no matter how impartial they are--go into it thinking, "Well, if they arrested the guy, he must be guilty." And you end up having to prove your innocence. Often this is where a good attorney comes in and she or he can frame the evidence for the jury--pardon the pun. Since most arrestees can't afford a good attorney...they lose.