First, the two are not exclusive. One can introspect and go straight for plan B without bothering with the PIP. Why go along with the PIP even if you accept you are at fault, if you do not know if the company will keep you? You can always improve at a new job.

To me, it does seem like a prisoner's dilemma.

You have to weigh the probabilities of each outcome and decide. In the few cases I've seen, the real issue was a disagreement between the manager and the employee on whether their was underperformance at all. As a clear example I've seen in my company: The employee is more productive than a coworker, yet he was put on a PIP and the other wasn't. He also did not work long hours, choosing instead to be efficient. He was in the habit of disagreeing with his manager and the customer on many issues. However, the PIP did not mention any of the last part - it just said he was slow to meet his goals and was often not at his cube. Yet when he went to the manager, HR, etc and asked what "slow" means and how it is being measured (because he likely was ahead of some of his coworkers on those metrics), etc, he really did not get anything concrete - just phrases like "well if your peers and manager think you are being slow, that is enough".

This kind of example is not unusual where I'm at. And this is an example of no transparency. The employee literally was excused of something but had no way to defend himself: No evidence presented by the management that he could question. Just that his manager and customer were not happy with his speed, and some of his coworkers saying he would leave work before the others.

When you see that happen at your company, you lose faith in HR. If I get a PIP that I disagree with, I'm not going to bother. Plan B will become Plan A. Heck, even if I agree with the PIP, I don't see a point in following it through, because given the company's policy, I have no way of knowing if they are sincerely trying to help me or trying to get rid of me. I can always introspect and go for plan B.