[Lazare]

The explicit grant was revoked in circumstances that are unlikely to matter for the overwhelming majority of users.

The implicit grant doesn't get revoked, but how it works is still unclear. This is true especially (I believe) when it comes to sub-licenseability and patent exhaustion, where the exact mechanics have yet to be worked out in the courts. If I download MIT licensed code, and then distribute it to you, you have a valid license to the copyright on that code, despite the fact that you didn't obtain it from the copyright holder. Do you also have a valid license to any patents on that code? I have no idea, but I know enough to know it's not as obvious an answer as you might guess.

In general, explicit is better than implicit, and settled legal questions are better than fascinating mysteries. These factors weigh in favour of an explicit patent grant. The terms of the explicit patent grant Facebook was using, granted, were quite negative, but on balance I think I (and my current employer) probably were marginally better off with the older license terms. YMMV. :)