Cross posting from a KillTenRats comment I made.
Why should developers bother with a true "open beta" at all? Instead, they can use beta invites as leverage for pre-orders, funding from partners, contests to drive traffic and create content… etc.
Why let people in to an unfinished game, for free? You should at least weasel some good for yourself out of the deal. Beta players are generally just going to judge you on your flaws and move on.
Sure, there was a time when open beta was about testing, but that’s long since over. Generally (ignoring the hardcore fans) people will put up with the same number of bugs in beta as they will post launch. The game is either fun or not fun, and if it's not fun in beta very few will come back after launch.