I can see why a public test server doesn’t exist given Anet’s game design philosophy. Revealing new one time event content before it goes live would diminish the sense of wonder of said content. That’s fine and understandable.
What is not fine are the persistent bugs that go unfixed for weeks after a content patch is released. Recruiting players to play test dungeons and dynamic events and then fixing the majority of the bugs they find prior to releasing the live build would improve the game experience for everyone.