Betas (open, and closed too, to a degree) these days seem more like, "let's subtly have people try our product for free, get hooked and generate buzz while at it". It's the same as a free trial (although calling it like this has less positive response, it seems). Just another marketing stunt, long gone are the days where beta was used strictly for testing.
On the Web 2.0 environment, who started this? Was it Google with Gmail, or was there something else before?
Also, lately, at least in the MMO scene, going gold doesn't mean that the game is polished and ready for release, it just means that you're ready to charge customers.