The beta tag is one of Google’s brilliant hallmarks.
The beta phase (it can actually last forever) solves one of the key challenges in software product development. The first challenge (unsolved by the beta phase) is to build software that solves a serious consumer or business problem, which is no small task. But the second challenge (once your software solve a serious customer problem) is that you can never figure out how human beings will actually use your software once they get their hands on it. That is, unless you have a strong beta program and you watch a large number of your users in action, and you learn from them.
Google is the master of using emergent data. Don’t design Gmail. Let your beta testers play with your new email system until the right patterns emerge, and let them design Gmail for you. Same with Twitter. Let the right features emerge, don't airdrop them in from corporate HQ.
Having a strong beta culture is all about knowing when to stop inserting your own preferences and opinions into things, and letting the end users tell you how the product should be designed. That’s when the software gets designed right.