The idea behind Test Fest is twofold: firstly, to give an approachable way for PHP developers to contribute back to the core - we'll learn how to write tests on the day and can continue to do so after the event of course! Secondly, PHP itself gets better tests, and the language itself gets better. All in all, I can't see a reason not to do it!
There are events right around the world so if you can't be in Manchester then take a look around and see if there is something nearer to you that you could be involved in. Northen people - I'll see you there :)