As I was watching Return of the Jedi, and hearing Yoda talk to Luke about avoiding the mistakes of Vader and not letting the dark side dominate, I thought about how little doses of dark side seemed to help the Jedi defeat Sith.

Let's take a look at some of the Jedi vs. Sith battles...

Episode I
Maul was pretty much handling Qui-Gon and Obi-Wan. After getting the Jedi seperated, Qui-Gon calmed himself while waiting. He then resumed the battle and got killed. Obi-Wan, feeling anger against Maul upon seeing him kill Qiu-Gon, split Maul's saber and came close to winning the battle of the sabers.

Episode II
Presumbly, Obi-Wan, Anakin, and Yoda all fought in the Jedi way and either lost the Dooku or it was a draw.

Episode III
Dooku took out a Jedi-acting Obi-Wan and when Anakin took Dooku's advice and used his anger, he easily took down Dooku.

While not explicitly angry, Mace was aggressive and unforgiving towards Palpatine. He seemingly had defeated Palpatine with that aggressiveness and was ready to kill him.

Yoda, fighting without anger, couldn't defeat Palpatine. Obi-Wan, while not obviously angry, in my opinion had anger towards Anakin. From "Let her go" to "I will do what I must" to "Don't do it, Anakin" I think Obi-Wan didn't have the detached calmness a Jedi would be expected to have.

Episode V
With Yoda's teaching against the dark side fresh in his mind, Luke never seemed to get truly angry at Vader. And he lost.

Episode VI
Luke pretty much fought Vader to a draw (and Vader might have been toying with him to try and turn him). It wasn't until Vader's comments about Leia that Luke got angry and aggressive. It was that anger that seemed to drive Luke to defeating Vader. Luckily for the Rebellion, Luke let go of the anger. Perhaps it was that letting go of his anger, not finishing off Vader, and his committment to his Jedi father that let Anakin destroy the Empire.

Obviously I don't think the dark side of the Force is preferable, but it does seem to me that a little darkness can help a Jedi Knight defeat a Sith Lord.