Who's to say studying the personality types before taking the test won't influence your answers? For many there is quite a disconnect between who you are and who you _think_ you are. The difference is even greater between who you are and who you want to be. I'd bet people who read the prose before taking the test would alter their answers (even if subconsciously) to conform to who they want to be.
Tests like these can help you gain valuable insight into your personality and may help you get a handle on others with whom you don't click. If you game the test, you totally miss out on any knowledge it may provide.
I'm not sure it really can help you gain valuable insight into your personality. Has anyone measured that value? Do people who have seen their Meyers-Briggs results do better in some measurable way from people who haven't?
I think the benefit is less in helping you understand your own personality, and more in helping you understand how others' personalities differ from your own.
Tests like these can help you gain valuable insight into your personality and may help you get a handle on others with whom you don't click. If you game the test, you totally miss out on any knowledge it may provide.