I disagree almost entirely. Public betas are useful for rooting out edge case bugs that don't happen to everyone, or even a large number of people.
The bugs in iOS 8 have been apparent to every developer (and presumably Apple) since the very first beta. They just didn't fix them all.
Yosemite is becoming SO bug-free the public beta program really helped because ML and Mavericks were both disasters when they were initially released.Yosemite? It's so damn bugless. very happy with it.