Some lawyers argue that BSD/MIT includes an implicit patent grant, but this has never been tested in court. Facebook's lawyers obviously don't think it's clear that BSD includes an implicit patent grant, which is why they have an additional license. I don't see how "we hope that a judge would interpret the licence as an implied patent grant" is better than an actual patent grant (with some constraints).

Also, IANAL, but I never understood why the revocation of the explicit patent grant would cancel out the implied patent grant. If your argument is that "the BSD licence text implies a patent grant", shouldn't that be true regardless of any additional licenses?