[uDevil]

No, it's definitely not required. It is helpful to track your play, analyze your stats, and to replay and study hands. During play, it is helpful with a heads-up display (PokerAce, Gametime+) to categorize your opponents, especially if you multitable. If you only play one or two tables at a time, and you pay attention and take good notes, you can probably do just as well without it as with it.

I don't think most of us get nearly the benefit from PokerTracker that we could, either because we don't know how to use the information it gives us, or we're just to lazy to do the study and analysis.