Depends on your type of product and buzz you can generate imo. I mean, if your doing something new, and its practical that you have a limited amount of users, then a beta is great and make sense.

But most startups can't even generate the kind of buzz needed for a private beta, where it becomes worthwhile to build an invite/beta key system. If your product were to go public tomorrow, would your servers be killed in a traffic frenzy? If the answer is no, then you probably don't need a private beta.