i have worked at places that never used a PIP. they were fine. they did due diligence when hiring and helped employees grow. maybe i live in a diverse area, most people are socialized pretty well after university and a few years of employment. so disagree about never using them being a red flag. maybe at megacorp inc, when you're hiring thousands, it's a fact of life. with terms like "unregrettable [sic] attrition", you'll forgive me for being cynical.
none of the PIPs i've seen have been reasonable, but small sample size. i have also luckily never been on the receiving end, only observing from afar. hell, sometimes idiot managers have put their best, most helpful employees on a PIP because they're... spending a lot of time helping other members of their team! so their individual performance goes down, but the team performance goes up. but when you view people as "individual contributors", you don't see that.
i'll happily agree with you on "a strong indicator of poor management is a failure to effectively manage performance", and go one further, a stronger indicator of poor management is a failure to effectively evaluate performance. unfortunately, that is exactly what makes PIPs so tricky. ignoring who's perspective is most "correct", there's still a divide in either performance expectations or perception. the better strategy is still to leave, not see the PIP through. which makes PIPs a good signal for "you're about to be fired", but a bad signal for "we disagree on performance", simply because that difference in perspective exists. putting it on paper is again not necessarily helping the employee. if "it just doesn't get through", is a PIP going to change that? if they aren't performing, if "it just doesn't get through", why not fire them? a possible answer: passive aggressiveness ("work with an employee to correct performance issues") and leave a paper trail aka. ass-covering ("the needed improvement doesn't occur").
but look, i'm not a manager, and don't want to be. could i fire someone? probably not. so is a PIP more humane? i think so. at the end of the day, it sounds like we both agree what it's for, just that you didn't agree with my phrasing. fair enough, although i still maintain from an employee perspective, assuming poor management if you get PIP'd once is a good plan. if you start seeing a pattern, then it's time to look in the mirror.