I think you're missing the plot. Currently, there is a big philosophical debate on whether OSS licenses like the BSD include an implicit patent grant. When Facebook explicitly includes a revocable patent grant, your argument of the BSD license "with no patent grant" is self-fulfilling. But if the PATENTS file had never existed, then what you have is "a BSD license with a potentially implicit patent grant", which has never been tried in court so could set a precedence, but is waaaay better for the consumer than a "BSD license with no patent grant because that patent grant has been explicitly granted and now revoked".