stack ranking actively hinders managers from retaining exceptional performers. if you only have 2 A grades to give out and you have five people deserving of it ALL of them are going to be motivated to switch to lower performing teams or entirely new employers.
any sort of 'objective' evaluation of employees is prone to being gamed and any gaming of evaluation is actively harmful to corporate culture/performance. witness google where lower profile or troubled projects are abandoned en masse by any employees with the ability to turn them around