People say stack ranking is bad within Microsoft because you're not ranked against all 50k+ employees but against 10+ people that report to the same manager.
If 2 out of 10 people are guaranteed low ranking, then it's better to be a mediocre programmer in a team of poor programmers than to be a very good programmer in a team of even better programmers.
It also means that it makes sense to sabotage the performance of your 9 colleagues because you only care about looking better than them, not about doing best possible job for the company or your team.
A global stack ranking, if it was actually possible, would be great. You do want to get rid of low performers (and hire better replacements) and in a large enough pool the bottom 20% will really be worth replacing with better people.