I'd go the other way for big organizations like Apple and Google -- they need to do public testing, because the bug that impacts 0.1% of their users will impact hundreds of thousands. And no QA team, regardless of how they're empowered or resourced, will find every 0.1% bug (or even every 1% bug). You need broad beta testing in those cases, just to get enough hands on.
There is a need for public betas, but a private QA team could catch a lot of bugs before release, and catch regressions. The public beta builds might not be much better, but they'd likely at least come with reported known issues.
Of course, iOS was nototorious for not alarming on January 1st because Steve wanted to sleep in (actually I don't know why, but I think it happened at least 2 years in a row, and maybe three)
