I say do it. It's great public speaking experience, if nothing else. If you want to do litigation or appellate work, it can be invaluable. It looks great on a resume, and if you're interviewed by people during your career who did moot court, it's an easy thing to have in common.
As the previous poster said, don't take it too seriously, and try to have some fun with it. You might enjoy it more than you did for the try-out competition, since it's a different sort of competitive atmosphere.