Their advantage is companies are willing to contribute

Hi :)
Adding their own stuff outside of one of the core projects doesn't necessarily mean feeding those additions back into either core project. Of course if they don't feed it back in then they have a much tinier number of people doing QA and then beta testing so it's a bit of a balancing act for them.

From a quality pov it's better to add "upstream" directly into whichever project but from a "keeping the secrets" (proprietary) pov it's better to keep their additions secret.

Both are fine and valid but only 1 really counts as contributing to the project.
Regards from
Tom :)