If React didn't have an explicit "grant" there would be an implicit grant. Is the explicit grant better than an implicit grant? It isn't because the explicit grant has what is known as a strong retaliation clause.
More info on weak vs strong retaliation clauses: http://www.rosenlaw.com/lj9.htm
React's patent "grant" gives you a license to React's patents. This sounds like a good thing, however, the "grant" has a "strong retaliation clause" which says that if you make any sort of patent claim against Facebook this patent grant automatically terminates.
Which means Facebook can now sue you for patent infringement for using React. You may think this is no worse than not having a patent grant at all. But that's not the case. If React didn't have an explicit patent grant then there would be an implicit grant which does not have any retaliation clauses, and cannot be revoked.
If you work for a software company and your company has patents then keep in mind that by using React you are giving Facebook a free license to your entire patent portfolio.