Companies come and go. If one company that contributes its own patented tech to a project like LLVM gets acquired by a troll, what protection users would have?

I see no reason to trust a company to behave responsibly towards a community if and when it no longer needs it.

Your insistence in stating the obvious - that nothing protects users from third-party patents - while refusing to acknowledge the question of how much BSD-like licenses protect users from patents owned by those who contribute code to BSD-licensed projects is disturbing.

It's not that I don't acknowledge that your emperor is wearing pasties, I just don't think they cover enough to be considered clothing.

I think the onus is on you, as the advocate of this clause, to name one actual patent troll case that would have been prevented by it. SCO wouldn't have been prevented. The recent Oracle action against Google would not have been prevented. So which one?

You need to provide protection from actual, plausible danger to justify the doubt, uncertainty, and fear that you are keen to spread.