I think it really depends how the employer sees a PIP. Some companies clearly use it to manage people out, for others it is a genuine effort to bring performance up to standard (but if it fails you're on your way out).
I've been on the other side of them twice. Small comfort I know but neither I nor HR liked it, there aren't that many people who enjoy seeing the worry and upset of the person placed on one. To be fair both individuals had been given huge amounts of help, support, mentoring, additional (repeat) training before we resorted to this.
One just really wasn't capable of reaching the standard required for that particular role, despite years in the industry. He eventually accepted a move to a less technical role in another team. Not great but not sacked!
The other was capable but would improve until we stepped back a bit and then take the foot off the gas. She was absolutely taking the piss though and got to written warning stage then left rather than be fired.
I know of two others in my department who came out the other side and appear to be performing well so it's not hopeless but again, it depends on what the company is aiming for.