Jack Welch popularized this decades ago, where every year he would force departments to cull 5-10% of their employees. He stated that this was well-received for the first few years, but after that it got harder, because managers were starting to let go people who were actually decent. But Welch insisted on this because it kept things from stagnating, as well as keeping open reqs when you found very good people. He also said that managers would start to try to game the system where they would hire poorly performing people just so that they could be culled.

I think there is some value to occasionally culling your worst performers, but having it as a practice every year the way Welch did, and presumably Google does, makes it tough on everyone because it could lead to situations where good but not outstanding employees are let go for no apparently reason.