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.