The only large (200+) private-sector company where I've worked is Google, where their stack-ranking/Perf system had some serious problems.
The first was a 5% cut-line. If you were in the bottom 5% of the Perf stack, you ended up on a PIP. Who usually ends up here? Some were objective underperformers, who'd been coasting for years, in some cases running at less than 1 CL per month. Most, on the other hand, were junior people on underperforming teams-- and of course, those junior people had the least to do with that team's underperformance, but they were poorly established so they got the shaft. That's how most layoffs actually work: junior people who landed on the least effective teams. It's not atypical, but it's dysfunctional.
By the way, PIPs have nothing to do with "improving performance". They exist to make it easy to fire people without severance. I actually think PIPs are imbecilic, because I'd rather cut a 3-month severance check and separate cleanly than keep a fired employee in the office for 1 month, but that's another rant. PIPs make the finance department happy (look at what we saved on severance payments!) but piss all over morale and make the manager's life hell, to say nothing about what they do for the PIP'd employee.
At Google, a PIP is effectively permanent. Even if you pass the PIP, you become damaged goods and can't transfer. It sticks around on your permanent record until you leave.
Which brings discussion to the second problem with Google's system. It sticks around forever.
When these types of systems get rotten is when they start getting in the way of peoples' transfers. It means that the people who really should be moving to other projects can't get on to them because no one wants to take a chance on that guy who was an "objective underperformer" in Q4 2005.
So, to come around to your point, I agree that if the rankings are only used to adjust pay, there's not much harm done. Unfortunately, it's rare that data collected is only used for the stated purpose. If these numbers start damaging peoples' ability to choose their own projects, it can become nasty quickly.
