No, you'll only lose the patent license for React, leaving you with exactly the same copyright license you would have had with plain BSD. There is no scenario in which the PATENTS file (not clause) enables Facebook to infringe on others' patents any more than they would have been able to do with plain BSD. If you think this makes using React risky, then you'd also have to believe that using thousands of other BSD-licensed projects has been risky for years. That is, if you want to be logically or legally consistent.
This "adds risk" argument is very familiar from other FUD campaigns (e.g. Microsoft's) against open source in general. It's pretty weird to see people who claim to believe in open source pushing arguments that work against most open source.
Seems that the real issue is that Facebook asserts/implies they have patents relevant to React, and that they are willing to enforce them in court under certain conditions. This creates an uncertain situation. The patent grant helps somewhat, but limits your freedom of action should Facebook e.g. steal your technology. Look at the four possible scenarios:
Facebook | Your Company | Outcome
Grants patents conditionally | Does not sue Facebook | Things hum along nicely
Grants patents conditionally | Sues Facebook | You're liable for patent infringement
Doesn't grant patents | Does not sue Facebook | You're liable for patent infringement
Doesn't grant patents | Sues Facebook | You're liable for patent infringement
The patent grant is fine if you think you'll never, ever want to sue Facebook. But should a dispute arise, you'll quickly find the technological rug pulled out from under you. You've now built your entire business on Facebook's asserted IP and have no recourse but to license it or try (futilely) to defend your infringement in court.
This "adds risk" argument is very familiar from other FUD campaigns (e.g. Microsoft's) against open source in general. It's pretty weird to see people who claim to believe in open source pushing arguments that work against most open source.