I've managed tons of people any only used a formal pip with three, that I recall. Two were people transferred from other teams to see if I could retain them because they were about to be let go. Another was someone I hired who was obviously going through some type of mid-life crisis about six months in... One if the first group took direction and turned into an ok employee with lots of coaching. The other two had to be let go.

The first guy just never worked, he was collecting a cheque until he got fired. He was hard to deal with because every time I addressed his lack of any work product he would act like we'd never discussed this before. Just a really weird guy. I think he'd dragged out other jobs like that and figured I was too nice to let him go?

The second guy was similar. He slipped up a few times and it became obvious he was working two jobs at the same time. He admitted it and said he was good enough to do all his work in half the time a normal person would. Great but his work product was complete crap and delivered late and incomplete every time.

I have no idea how many people I've managed or supervised in some capacity or other over the years. The vast, vast majority are doing what they can and if they get off track they just need a hand getting back on track (there's often something going on outside work that distracts them, this is usually temporary and a little understanding and accommodation goes a long way - we're all just trying to get through the day, sometimes that easier than others).

At the company I work for a PIP is the last straw. If you get put on one you've already been written up several times and had your failings explained to you (these writeups go through HR for approval to make sure they are clear). If you make it to the point that you're being put on a PIP you're probably a lost cause at that point anyway but it's a way for a manager to stave off actually firing you if they think you might be salvageable. I've never seen anyone make it through one but I would have fired them long before that if it was up to me.