I get that there's legal uncertainty: I can imagine some companies, especially larger ones, not wanting to give up their patent-suit option just because one random small team in their org is using React. And barriers to adoption aren't a great idea when one of your goals is wide adoption.
Overall, though, I think making it much harder to file patent suits -- at least for software patents -- is a great thing. Facebook's (and others') implementation of it might have left something to be desired, and they're certainly a large company with a lot of resources behind them, so the power dynamic might be a bit lopsided, but...
Well, let's just say I'm a small company or individual who wants to release some open source code. Doing so under a BSD+Patents-style license gives me some protection from any big players who decide to use my software. How is this a bad thing?
I suppose I hold a pretty strong anti-patents stance, though. I'd be happy if (most?) patent protection just went away entirely.
As an aside, note that it looks like FB is going with straight MIT. So they're not even giving users of their software a patent grant at all anymore[1], and reserve the right to sue you later just for using their software, if they happen to have a patent on something in it.
[1] Yes, there are some legal opinions that the MIT license's language includes an implicit patent grant, but I don't believe that's a settled matter.