In the places I've used PIP's we used them mainly on staff we hoped to get to a level where we could promote them. We didn't waste time on proper PIP's for people we wanted to manage out - for those people HR simply mandated no raises ever. They'd leave or gradually get cheap enough for what they delivered to be worth having around.
EDIT: It's worth noting that in these cases the staff in question also had clear evidence in the form of signed reviews rating them that they were performing well. E.g. the guy I spent most time on a performance improvement plan for was ranked "exceeds expectations" in every criteria three years in a row while I was there, and was given 15%+ raises each of those years. This was in Europe, but for a US company.
