Could you make a compromise and agree that the terms were ambiguous enough to make people feel uncomfortable or uncertain about whether or not it was safe (or felt safe) to architect an application on top of software whose license could be revoked without wrongdoing? That would not require the terms be legally enforceable, thus a debate on validity could be considered moot.
I truly have no idea, but I'll default to charitable interpretations of both sides and assume that people had genuine concerns and that Facebook didn't intend anything nefarious.
In which case the change to the PATENTS file is a net win for everyone: Facebook can now clarify that they had no nefarious intentions, and everyone else has peace of mind.
