From what I learn about public test server, at least on XI, is to check latest feature before SE decide to release it in live server, or withhold it. Cait Sith summon and new UI are among those that's taken longest time being in test server, with Cait Sith summon being recently and officially approve for public live release after years being in test server and ppl already doubting it would see the light. Basically, test server serves as to test new features whether it's ok or not.