Basically, the grant guarantees that you can't be sued if you don't sue first. No grant means that you can be used in any case.
Not just that, but if Facebook patents covered any similar framework (eg Vue.js) they could still sue you if you use that other framework.
I have head a theory that the BSD licence has an implicit patent grant. If FB sued you for using a plain BSD licence, you could argue the BSD covers it, if the courts agreed, you'd be fine.
One objection to the BSD+Patents licence is that it sorta removed the ambiguity, and explicilty implies the BSD has no patent aspects.
Even the experts admit that it's untested, and every law student knows that anything up to an actual test in an actual court is little better than a guess. Also, beware of "experts" who aren't even talking about copyright law. For example, consider the three cases mentioned in the "US" section here.
http://en.swpat.org/wiki/Implicit_patent_licence
The Hewlett Packard quote is clearly about products, which I know for sure are not legally the same as licenses. The Bottom Line quote's mention of purchases casts a similar shadow. Only the De Forest quote passes even a rudimentary sniff test. The case for this implicit license is weak indeed.
But fine, let's say there is such an implicit license. I have good news for you: it would apply to the Facebook code as well. The same principle used to argue for the existence of an implicit patent-license grant under plain BSD - i.e. that trying to bring a patent action would interfere with the still-operative copyright permission given in the LICENSE file - would still apply. Nothing in the PATENTS file would change that.
Belt and suspenders still beats belt alone, no matter how strong or weak the "implicit license" belt might be.